Rubber o-rings are exotic, but well worth the effort if you're running crazy high compression or revs.
There are no heads I know of for Puch that come standard with an o-ring.
Basicly, what you want is a head with a seperate chamber. Meaning that you can replace the combustion chamber as you like. These are not available ready made, and have to be machined.
The + about this, is that you can have a cylinder with a deck of, say, 2mm and you can make a drop in head with a rubber o-ring on the side. A drop in head gives you the ability to tinker with compression ratio and squish, while keeping the rest of the engine the same. Drop it down, get higher compression and lower squish. Take it up, get higher squish and lower compression. And you can alter the shape of the chamber pretty easily, all you have to do is have a new insert machined, instead of having a whole head modified again.
The downside is that these are a lot of extra variables to work with, and that those kinds of heads don't come cheap.