You're actually referring to the primary compression, not the secondary compression.

Consider filling in the space behind the reed valve stoppers. That's one area that gives good results. Then just tidy up the cases and get rid of sharp edges and what have you.

Remember that the water pump and 2T oil pump are also driven off the primary gears, so unless you're going to be running premix and an electric water pump then the waterpump shaft gear also needs to be considered.