Over the years I've found out with a typical "sport" aircraft engines that less-is-more when it comes to modifications. At the r.p.m. the airplane engines turn, compared to the car engines, you don't want to remove too much material. (it can slow down the flow velocity, and that is not good for low to mid range operation )

I typically put a radius on the sharp edges that are in the fuel/air flow path.
A radius on the lower edges of the F. and R. transfer port openings (on the outside of the sleeve ) and at the very bottom of the sleeve.

A bit of a bell mouth or radius at the crank opening into the case...maybe a "turbo cut" or two if you want to get ambitious. (look at the internals some r/c car engines if you want some ideas there ) Use a short piece of brass tubing over the crank pin to keep from accidently nicking it.

If the rod is a simple rectangular X section, then a radius on the edges...(the current TT .40 and .46 rod is a nice example of what you could imitate )

Sometimes I will work the piston skirt, it depends on if it masks the transfer channels too much while at or nearing BDC...removal of material under the wrist pin....but this is getting a bit into the "tricky" areas of modifications. If you have new wrist pin retainers, and want to experiment...

Magic Marker (blue or black ) work great for machinist "bluing" if you need it.