Unlike most other bikes, the cranks on the big BMWs are extremely solid and failures or wear are very rare, same with most mechanical items.
It means dirt cheap parts second hand, a friend is replacing his K1200 crank- previous owner failed to service it.
The crank in perfect condition only $150 from a wrecked bike. Try finding a jap bike crank or Harley one for that price.
Even better, you can remove the crank with the engine in place and box still attached, takes 30 mins to remove. Crank is driving a gear to the flywheel, just remove sump and unbolt the mains. New bearings, for crank and seals, total job under $500.