This should be a very easy refurbishing. As long as there was never any ethanol used, a flush of the fluids should have you riding in short order. I would recommend running carb cleaner with the first fuel to helpbreak up any varnish deposits.
I recently took possession of a 1975 R75/6. A very good friend of mine was the second owner when he bought it in the late seventies. We rode together for a few years then due to family, etc I sold my bikes and he parked his Beemer in a garage for the last 40 years with 11,800 miles on the odo.. A month ago he gave the bike to me and I am in the process of resurrecting it. The question I have regards what needs to be replaced. I’m already planning on replacing everything that’s rubber, fuel lines, brake line, etc. I’m thinking that hard bits, carburetor needles and jets, valves, valve seats, etc will need some cleaning/degunking and lubrication but shouldn’t they otherwise be good to go? I appreciate any guidance, By the way, one of the great things about being stored indoors the whole time is every nut, bolt, clamp, etc I’ve dealt with has been a breeze to get off.