KDX - I have had several of the older RMs - ranging from a 77 125 I owned "back in the day", to a 76 370 I restored a couple years ago, a 77 370 I am in process of restoring right now (only to turn around and sell it to help pay for my new KTM...bummer) and a few others I have parted out already or currently have various parts for. (I have some 125 stuff BTW)
One good thing with "most" RMs from 76 to 78 - many many parts are interchangeable (not all so be carefull). 78 had more significant changes than 76 and 77, 79 was the BIG change year.
try vintage-suzuki.com for repro plastic, seat covers etc. he can also get many many other items. good guy to deal with as well. He has most of his stuff on evil bay. NOS stuff can be found, but it takes some digging - try evil bay.
best of luck to you - these old bikes are addictive and wallet draining!