I can't speak for other builders, but despite having some crappy offshore machine shop equipment, MIG and TIG welders, a 20 year collection of BMW parts, and having done all of the work myself thus far except for surfacing of the bell housing and machining the flywheel to accept a BMW clutch, I've got $8K+US tied up in the bike. All that's left to buy is a set of tires.
Regarding conversions, personally I didn't go that route because everything is a compromise. I plunked the engine down on the floor and built a motorcycle around it. The challenge in the eyeball designing approach is that it's like putting a complete jigsaw puzzle together in your head before touching the pieces.
As far as BMW donor bikes, I didn't use any one bike, but parts from various models. If you re-read my posting of February 2007 you will find detailed info there on BMW donor parts.
Regarding gearbox modification for higher top speed, browse the posts - there are several folks who have successfully modified BMW/Guzzi gearboxes.
A question often asked is how to build an engine adapter. The following pictures and comments show how I built mine. No magic and no secret, just many, many hours of hard work. There are various approaches. I built mine from aluminum, and used flat bar welded together bcause it was much cheaper than buying a piece of aluminum plate.
Something tells me I posted this once already but I couldn't find my previous post. Anyway:
1. Ring large enough to enclose the flywheel, and pieces of aluminum flat bar for each side of the ring
2. The flat bars will be welded together and then welded to the ring to provide the mating surfaces for the engine and gearbox:
3. The flat bars for the gearbox side are trimmed to the OD of the ring, size, those for the engine side are cut for the ID of the ring:
4. The flat bars are welded to the ring. The hole is cut for the gearbox and the gearbox mounting holes drilled (shown in yellow):
Finally, the adapter is temporarily attached to the engine, the engine mounting holes are marked and the facing is marked for trimming of the excess material, and presto - you have an engine adapter. See, there's nothing to it!
