Parts list,prices,and diagram(exploded view)

You have not mentioned the diagram requirement for fishing rod or reel but Baitcastingfish is the platform that might help you in this regard as I have found two useful links for you:
Labelled diagram of the parts of fishing rod:
https://www.baitcastingfish.com/explore-parts-of-fishing-rod-and-their-types/
Labelled parts of fishing reels:

https://www.baitcastingfish.com/what-is-baitcasting-reel-explore-parts-of-baitcasting-reels/

Mike Reel Repair Fishing Reel Parts Service

Order Reel Parts
Mike Reel Repair Fishing Reel Parts Service
http://www.davesreelservice.com/
Mat Reel Repair And Parts

OKuma is very helpful with their center pin reels.
Here's a link to their site for the center pin reels.
Okuma Fishing Products Buy Reels
open the "view models" tab then click the schematics tab for the aventa. This will open an exploded view of the reel so you can identify the parts you need. Then give them a call.

Sounds like your drag is too loose. Best way to get used to setting a baitcaster, in my opinion, is to put a 1/2 ounce weight on the end of the line and engage the spool. The line should drop to the ground freely, but not full speed - if too fast, the spool will continue to spin and you'll get that dreaded birdsnest. If it doesn't reach the ground, you need to loosen it more. Once you have it set, keep that weight on and go practice, using the thumbing process Parorae noted in the previous answer.
They take some getting used to for sure!

common problem especially with new users who are used to fixed spool casting, when casting with a multiplier you should keep your thumb over the drum so you can slow down the drum as your cast slows to fall in the water, if you had a drag washer you wouldnt be able to cast, you will get the hang of it with practice, but even with 30 odd years of use i still ocaisionaly get a nest.

There is a diamond bar (my name for it) that the guide slides back and forth on. There are groves cut in it. Make sure that nothing is stuck in any of the grooves. Clean them out thoroughly and relube the bar. Inside the guide there is a post that rides in those groves. If doing the above suggestion doesn't cure the problem the guide may need to be replaced. Currently their website is not completed yet and there isn't access to the parts diagrams. This is the address. You can check there later and see if the site is completed. http://www.pinnaclefishing.com/shop/t-parts-diagram.aspx

Run your line through the guide and tie around spool using a good double loop knot

google brand name for phone number of company and call them sometimes they will send u parts like that for free or try e bay