You may want to be really nice to your local parts man.
Sometimes upping the compression is as simple as using the stock head gasket from another era engine.
Normally when manufacturers reduce compression on an existing engine platform, (like the B-12) they do it by installing a thicker head gasket at assembly. (In fact this is the case with the Yamaha FZ-1 engine vs the
R-1 engine. The same is used in both bikes).
I have no idea if this is possible with the B-12, but I would bet a head gasket from an earlier Gixer Engine would be thinner. I have no idea how you would investigate this, but a skilled parts guy could help you.