I just bought a 2008 tank urban sporty 50 just a week ago and my speed went from 35 to 25mph in that time. is there some tweaks or mods that i can do to help me get better performance ? and if so maybe a website for parts or a tip site.

No, there is only one gas tank on a scooter of that type. You ALWAYS use premium gas, correct? Also there is a small hole in the gas tank cap. Air must be able to enter this hole. Make sure it isn't clogged and air can enter. NEVER let your scooter run out of gas if you can help it. Make sure the gas filter is clean and installed correctly (follow the flow arrows printed on it). Hope this helps!

The speedometer or the speed sensor are broken. Buy new parts here:
Speedometer:
http://www.woodfins.com/Parts/2008-TOYOTA-YARIS-83-R119-273-Speedometer_Instrument_Cluster.html

Tire balance most likely, I'm not a big fan of the work done on OEM tires. Also have them check the air in all the tires, even the spare. Most times it has 1/3rd of the recommended pressure.

Most likely it is a 4 speed automatic over-drive transmission. You can shift it thru the gears if you want a sporty feel, but not needed. The fog light bulbs may be burned out, a bad fog light fuse or switch. Does the switch illuminate when you switch it on?

Will you need any mods depends on what you're speaking of. The tank should fit without any problems unless Harley changed the frame for some reason.

Now, as for using a fuel injection tank on a carburetor equipped bike, I'm not sure what you will need to do. I have not worked on a fuel injected Sporty but I'm assuming that is what your talking about when you say "FI" in your post.

Check/Replace the vehicle speed sensor (VSS).

VSS generate a pulse count to the vehicle computer. If this sensor send out extra count the speedometer will read higher also.

Use GoogleMap to plan a short trip. Reset the speedometer and see what's the mileage count.

I was reading lots of forums and tech support pages on how to get my Cricut back online. I went to the store and got the Deep Cutting Blade & Housing and I got two replacement blades for the original housing that came with the Cricut. I also bought a pack of new mats. I replaced the original blade with a new one, used a new mat, and tweaked the depth and speed of my cuts and my little Cricut is whirling again!

water in the fuel.or dirty fuel.Try changing the fuel filter if no joy remove tank and clean.

The timing of the plug change is probly chance Your speedo is a cable run from your front axle spacer between fork tube and Bering a spark plug or a wire could only put out the speedo light not affect the speed sensors caus there isn't one one a 95 sporty I only know cause I took off every thing on my chopper but headlamp tail lamp and starter switch and I had to make a custom spacer to take the place of my speed sensor slash Bering after I ditched my dash years ago on my 93 evo sporty