Have you thought about transplanting a R1200S motor/ecu into it form a crashed bike? With one of those, you could at least ride it while you searched for the parts that you need. Then once you had all of the parts collected, you could possibly sell the R1200S motor for what you paid for it. Heck, you might even find that it’s just fine as a frankenbike.
Here’s a rough salvage R1200S that I’m guessing will sell for less than $1k.
Salvage 2007 BMW R1200 S for Sale
Put a R1200S Motor / ECU into it. FTW.