Distinct front fork knocking

With the bike at rest, tie a cable tie/zip tie on the fork inner hard against the fork outer. Ride it normally and then check how far the tie has been pushed away from the outer seal. If the distance is more than halfway along the fork I would suggest there is a problem. If not much movement then I would check your bearings

Your bike has had major changes to the geometry. You already know this and are probably very careful when riding it as a result. I am presuming you are not able to ask your dad for advice on this matter now. I am sorry for this. He must have been a very cool dad.

I dont think I would make any structural changes to your dads design unless you could revert to an earlier version by changing the fork. I would consider different tires, however.

Good luck with your project.

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Did you buy this bike from a dealer if so, I would take the bike back to them and state the problem. Ask them if there is a recall notice for this. Also inform them that it needs to be repaired under warranty as you've already had an accident while riding it.

Thanks for the inquiry!
From your information I can't tell if this is a off-road bike or a street bike. With the extended swing arm I'm suspecting off-road. In either case it's very much same issues. You have two issues and we will deal with them separately.
Chain catching sound:
In low gear when you have maximum torque applied to your rear sprocket & wheel many times the primary sprocket (smallest one under the engine side cover) will slip and actually jump teeth. You indicated this is the third season, so it's seen a lot of wear. It would also explain that when you adjust/ tighten the chain tensioner the noise is gone as the front sprocket is not slipping in the chain. If this happens a couple of times the front sprocket is ruined. Pull the front sprocket cover and inspect the teeth. The teeth will be smooth and ground away if this has occurred. Most off-road riders go through 1 - 2 sprocket and chain sets per season with the abrasion form the sand and mud if they are serious about their riding. Replace front sprocket or both sprockets & chain.
Other possibilities are that you have lost your chain guide or the plastic wear plates inside the chain guide are gone and the chain is banging against the metallic sides or your swing arm making the chain jumping sound you have described. Replace the guide or wear plates to solve this.
Whooping sound on front of bike while coasting: This familiar sound appears to be related to your front wheel. The best way to test this is to set your bike on a bench, crate, or test stand so the front wheel/tire is off the floor/ground and spin the wheel. Your rim may be bent allowing the tire to rub against the fork tubes or front fender making the whooping sound. The engine noise drown's it out under power but noise is audible when coasting with the clutch in and engine idling. The simple fix is a spoke tightening and adjustment to properly true the wheel. You will hear if its a bearing or something else being rubbed against by a wooble or something out of round (run out) causing the whooping sound.
I hope I've provided you useful assistance and that this information allows you to make an informed decision as to how to proceed from here. Please be sure to vote on my response. I hope I've earned a good rating from you today.
Please post a comment if you need additional assistance so that I can help you further.
Happy cycling,
TF

A "hardtail" is a mountain bike that has a front suspension fork (like the one on the right) and no shock absorber in the back. Good hardtails are light, fast and responsive, and because of this they are good for climbing.
Even the cheapest mountain bikes these days tend to have suspension forks. That's not necessarily a good thing, since most of the forks on cheap bikes are of poor quality and will probably need replacing before too long. The cheapest forks also tend to be sold only on new bikes, and not separately. Find out the make and model of the fork, and search around on the web for it. The price will give you an indication of the fork quality.
Most decent bikes are sold with Rock Shox, Manitou, or Marzocchi. Marzocchi has been making forks longer than anyone and has probably the best reputation, but they tend to be expensive so you won't find them on anything but expensive bikes. Rock Shox and Manitou make lower-end (but reasonably good) forks. Other reputable brands such as Fox and Suntour make forks that may very well provide a good service life, but are not as well known for mountain bike forks. Replacement forks can be found at very low prices during sales and clearance events, so they can be upgraded later (sometimes at a bargain price). A "full suspension" bike has the front suspension fork and a rear suspension (like the bike on the right). These are highly recommended if you ride in a lot of rocks, as the rear suspension allows you to glide over rough terrain. They're also good for big jumps. The rear shock adds some complexity to the bike, and a good bit more weight at any give price point, plus there'll be a bit more maintenance. Plus, you'll pay a few hundred dollars more than a hardtail for a bike of otherwise equal components. Depending on your terrain, it might be worth it; riding a hardtail in big rocks can be brutal.

Your front fork seal are bad and need to be replaced.
When you loose fork oil, your loosing suspension dampening and only riding on the springs.(POGO)

Replace both front fork seals and dust caps, and put same amount of fork fluid in both forks.

I bought my seals and caps for $30 on ebay, and then took my forks off my bike and took them to a shop and they charged me $75 to do the job.

You can take them the whole bike and they will charge $100 - $150 including parts to do the job.

There are three most common weights of fork oil. 5W is a light oil for a smoother ride, 10W is a heaver oil for stiffer suspension and 15W is mainly used for a stiffer ride such as racing. I have found 20W and 25W oils but they are not that common.
If you are a pleasure rider 5W or 10W would be the right oil for you. If you are always "in the twisties" and riding the limits of the bike, go with the 15W.

Tighten the head bearings as per your manual.
Ride Safe
Bike-doc

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a trace is normal, to much is not. that probable is due to dryed out fork seal, as they ere not getting lubed due to inactivity. the air fittings are for ajustable damping. if you ride 2 up or yourself being heavy or you just want a firmer fork suspension. i don;t know the range for your bike. try asking the dealer.or just put in 5 lbs. and try. if the presure doesn't last then you have leaky seals......