These piston and cylinder kits seem to be the rage right now. I used one on the last build and had very little problems aside from the pushrod seals being extremely difficult to install.
The pistons are made to mate with the R75/5 heads and you can use the same carbs (might need to change jetting).
Here is a quick look at the piston and how it compares to a stock R75/5 piston that i have lying around.
I haven’t heard of any issues with these yet. Supposedly they give you a 20% gain in power… so that makes it around 900cc.
I’m pretty sure they are cheaper then replacement BMW parts too.