I have built all three of Randys engines. Here is how it went with me:
5/16 of 4 HP horizontal - Governor took a lot of time to get right, was tricky for me.
5/16 of 4HP vertical - Could not get it to run with liquid fuel, fuel outlet is lower then needle valve. Runs fine on propane.
½ of 1HP horizontal - Went fine, no problems.
I thing I noted was the Al used in the castings, was brittle. Make sure to use a fillet in any corner
Ken K