What about the Guzzi in the background? That would be neat as well.
If that BMW has been sitting for 15 or 20 years with the top end off,the crankshaft and internals could be stuffed as well. Parts for those old beemers are generally available from BMW,prices were going steadily up and up when I rebuilt an R75/6 4 or 5 years ago,shocking is a mild take on what the prices were then. Used parts can sometimes be found on various forums.
You might be better off finding a runner of some sort,a bike like that could snowball into an expensive time consuming lesson .
I just picked up a 1974 XS650 yesterday for 100.00,engine turns over,bike is complete/uncrashed with some corrosion but mainly dirty.
Parts are available on ebay and here and there,Yamaha built a ton of them.
Those old beemers have street cred,but are a clunker to ride,crap brakes,crap shifting,handling like a rubber cow,and slow to the point of distraction. A guy can spend many years addressing all these problems,once a complete rebuild has been done anyway.
Airhead people are special people,one will check in any moment now.
Some bikes around at times