James, it's good to have a hobby.
I owned a Yamaha XS750 back in the day, obviously a very different machine but it has "family" styling.
I think that you may find the restoration of a 30 year old Japanese machine as somewhat more difficult proposition to a 30 year old BMW, simply put a hell of a lot of parts are simply no longer available, or are bloody hard to find.
To give an example. My Yamaha XT350 (well really 329cc, a story I may tell for laughs one day) needed new "rubber" inlet trumpets for its dual Teki carburettor (bastard of an idea that I hope is never, ever repeated, slide carb one side and CV the other). Said rubber inlets have been NLA for about 10 years.
I looked for alternatives without success. Although I did find a cheery person in the UK who has been making his own rubber trumpets for RD series machines for years, he very kindly sent me a great deal of material and spent some time on the telephone to me.
Just as I started to acquire the stuff to make my own I found a Yamaha shop in Nebraska that still had stock, albeit at $100 each please post. I paid in a heatbeat and the old girl once again ran well and with its usual hammer like reliability.
Other things cannot be had for love or money - suspension bushes (made my own from bronze), suspension link pivots - adapted later model TT to suit) etc etc.....
Lots of luck and have fun.