Back when R-series Beemers were cheap and parts were even cheaper...
I bought an ex-Polis R80RT with about 80K on it. Think I only paid £250 at the time.
WIthin the next six months the only things needing replaced were the battery (discovered a Reliant Robin battery would fit, for a third the price) and the rear wheel bearings - not entirely unexpected on any bike of that mileage.
They had a bit of a reputation for lunching the rear drive splines 'twixt pumpkin and wheel, but mine were fine.
I found a bloke selling the complete 1000cc kit from a crashed R100T, so happily fitted the pistons, barrels, and heads one weekend. The carburettors didn't fit because the intake adapters were different, as you'd expect.
I rode around like that for a couple of months and was reasonably happy with the conversion; definitely more power and coupled with the R80 lower gearing it was capable of giving a good account of itself in the London Grand Prix.
I finally found a set of R100 carb elbows to allow me to fit the proper carbs - an immediate improvement again. The butt dyno felt like it was being kicked by another 5 or 10 horses and the torque at the rear wheel was almost stump-pulling, because of that R80 rear end.
The amazing thing was that, as an 800cc, I'd get an average of 40mpg, in a mixed day - long motorway hauls, London traffic, etc. After fitting the proper carbs my average went up to 50mpg.
Alas, the bottom end, which was shared by all the R-series, wasn't up to the task of the 1000 conversion and it ran a big end bearing. That's probably why the R100 has a cooler and likely an uprated oil pump. If I'd had a decent workshop at my disposal I'd have repaired it and carried on, but just chopped it in against one of the worst bikes ever, but that's another story.
I'd have another R80 / R100 tomorrow, but the prices have gone stupid in recent years.
Those lattice-cast wheels were notorious for cracking - inspect yours carefully.