Jiminy Crickets that is bad luck! I've never seen a broken camshaft on one of these engines before. That is a big quote to repair - even by BMW (Bring More Wampum) standards.. Yes, an engine from an 81-84 R65 will fit, though if you have the older, heavy flywheel clutch and not the 81-> clutch carrier assembly there will be some additional clutch changeover parts that will be needed, unless you get a replacement transmission too.
The bike looks too nice to part out, in my opinion.