Tbere are three ways (at least) to reduce compression ratio. Mill the pistons to reduce deck height or dome, enlarge the combustion chamber (minimally effective) or a thicker head gasket.
If you have open chamber heads, and dont have a quench dome piston, you dont have any quench now to lose. A set of thick head gaskets can lower your compression quite a bit. Cheapest, easiest ,simplest solution.
If you post your calculations and engine specs that say you are at 11 to 1, we can better advise what we would do.
