What parts fit both the ttr90 and pw80?

I would imagine anything that is frame-related should work. Once you get into electricals and anything engine-related, it's probably a no go.

Heck, the plastics might even be interchangeable.

and I'm moving this to the mini-bike forum. Might get you some more help

I'm using BBR fork and shock springs.
Pro-Taper used to make a top bar clamp that accepted 7/8" bars and is adjustable fore and aft. Thay also sold mini Yamaha bars. These really stiffen up the front end.
I'm using TTR90 foot pegs but they did require a little grinding to fit.
Other than the front fender I think most of the TTR plastic is different.

Performance wise there's a thread on here that covers a lot of PW80 mods.