Nice work keeping those old quad parts out of the junkyard.
Was it a Yamaha ? I chopped up a 350 Warrior and made two projects from it, a kart/buggy (no cage yet) and a reverse trike.
There is potential that you may see an issue I am having with my kart/buggy since the engine is mounted to the monoshock rear assembly. (many karts have no rear suspension so the whole kart is a dampner)
The engine gets shaken a LOT on the monoshock, and I see gas leaking from the filler cap and oil on my motor mount. Not sure wh the oil is coming out, but I plan to relocate my fuel tank to the frame behind the seat so that gas doesn't shake out on the TAV. Also try to keep that belt dry when refueling because the friction drive is affected if the belt gets wet (mine ate the belt one time after a sloppy refueling)
I really like the floor pan and side rail shapes, and will be watching the thread. Good work !
<edit> Sorry for rambling on, but I saw a picture showing you have a TAV belt guard, that should keep fuel off it. Nice.