I had a '70 or '71 r75, wonderful bike, 4 speed if I recall. Big flywheel made the bike feel like it was torquing sideways when you got on it off the line.
I've had a '82 r100rs (/7) that I truely love for the past 3 or 4 seasons. It has about 15000 on it, but I have to do some front suspension work and perhaps pushrod seals this winter. I've had it naked for the last bit, but am going to reinstall the fairing for next season.
Good luck with your /5, it'll probably keep running as long as you do!
All best,
Ben