Hi militarymonark,
There are many modifications to enhance the vintage Whizzer motor. The "H" motor is slightly more difficult to extract power because of the smaller ports.
Mill .060" from the head. Remove the "island" in the combustion chamber. Open the intake and exhaust ports. Because of cylinder design the exhaust on the "H" cylinder can't be opened as much as later versions. Install a slightly larger carburetor, such as the Carter N703, N665, or Tillotson MT12A.
Install the larger valves from the "300" or later cylinder, the "H" has 3/4" valves, whereas the later motors used 7/8" valves. If possible replace both valves with the larger units, or at minimum upgrade the intake to the 7/8" size.
There are Weber heads, camshafts, and special carburetors to "hop" up the "H", however the larger valve, reworked combustion chamber, milled head, and larger carburetor will offer similar results for a lot less money.
You can also replace the original head gasket with a .010" custom copper gasket for even more compression.
The average "H" will travel at 35 MPH with an average rider [225 pounds]. The average "300" or later motors will easily exceed 45 MPH in stock form. Modified vintage motors have been known to hit 70 MPH on Dyno tests, however they were stroked, bored, special tri-pattern head [165 pound compression], special lifters, hi-lift camshaft, electronic ignition, Cushman MT series carburetor, and radical fuel blends.
I have built several hi-performance vintage motors, and the results were better than expected. I still have a few cylinders with a very large piston [Clinton], and oversize valves, but might be too much for normal use.
If you need help modifying the parts, let me know.
Have fun,