The "H" engines are for helicopter use. They have the big heat sink head. You probably wouldn't want one of those in a regular airplane.

The OS .46AX is the follow on engine to the .46FX. It has a slightly different crankshaft, wider forward transfer port which led to a 4-bolt head instead of a 6-bolt. This led to a different design cylinder and head. It also has a separate needle valve assembly instead of the cast-on one on the FX.