Extra head gaskets, or trace out out the one you have in the shim sotck.

First what prop are you using? Use a prop with 4 inches pitch or less. A 6in pitch prop+ lean setting = sore arms....

I use the older FP 40's and my setup is large size venturi, APC 11x4 or 11x 3 prop. Stock muffler pressure and a rich 2 cycle setting. Fuel with a little castor in it.

Try that first. Some guys grind on the head a little to reduce the compression, but I have never had to, an extra head gasket should do it.

There is a lot of mystery and lore about the OS 40 and what setup to use, most of the mods are attempting to slow the RPM down. I have been able to get it to run well in most cases with a flat props.