great bike owns one since new
you can find parts in UK and Germany but some are missing such crankshaft

to say nothing about crankcases.
check the gearbox before used it as it's the weak point of this bike . Other weaks points are the front braking (but if you fit a GSXR 1100 1986 master cylinder the brakes will be really good) and the size of the wheels cause very difficult to find good road tyres for RG 500 now (don't know why Metzeler doesn't built anymoyr ME 33 with soft gum

).
If the bike hasn't run since years check the kind of plastic disc which works with rotary disc, when the bike hasn't fire up since years they can break.