I just acquired a 99 Monster 900 and I am wondering what parts such as fenders, exhaust systems, huggers, etc... are backward compatible from the newer Monsters with little to no modification?

Define newer....

Post 696, very little.

Pre 696, a lot until 2000. After 2000 still quite a bit, but less.

Rotors, calipers, suspension (shock until 2000), dry clutch, front fender, mufflers/cans from *any* dss Monster etc etc

of the list you provided...none.

some of the parts from 00-08 are the same (not fender, hugger)
93-99 share most parts

jeff h
monsterparts.com