When I had my R50S, which was nearly 50 years, early in its life, it did have a problem with the head gasket leaking, between the two push rods by the spark plug.
But there was a dealer in North London, called frank Cox, and what might be called 'a character'. He was also politically, in the same mindset, and an admirer of Uncle Adolph, with the photographs at the back of the shop, and the huge collection of what might be termed regalia of from the period 1930s - 1945
However, he did know BMW's.
I hope no one is offended by these statements, that is not the intention, that was then 1960s, not now
He rode R50S vary slightly modified in very many long distance races, the Thruxton 500 miles, with a co rider. Did not win but always finished.
He had the same problem with that engine, and had cured this by making some gaskets from copper sheet, it may have been perhaps 2.5 millimeter thick.
He gave me a pair.
These had to be what is called annealed, to soften the metal. heat it with the propane torch, till it metal glows red, then plunge into cold water this you do every time you remove the copper gasket. Thus you only need one
That's certainly cured of the problems I had with the R50 S
You then bolt it down with a little grease on it, no joining compound.
Might be worth a try?
I don't have the gaskets now.
My R25, which has an R50S piston, has a gasket, I made many years ago from some material that was used for joining gas mains, large diameter pipes.
It must be 2 -3 mm thick and quite hard, but, with just a little grease upon it. It works, every time.
Just an idea of some of you may wish to try with copper gaskets