I built a version of the Panther Pup. I suggest you do not build it. There are many flaws with the engine, and there are many much better plans around for similar size engines.
Among the problems I see are these- even Bill could never run his for more than a minute without overheating. Mostly I think that was because it was brass and air cooled.
It has no oil seals on the crank or camshaft, so it throws oil all over. It also has no oil pump, just counts on splash in the crankcase.
The rocker arm is a very poor design- he has it as a tall T shape, pivoted at the base of the T. Than means it does more wobbling side to side than pushing down on the valve.
There are better designs around, Seal, Sealion, Mastif some of the Wall engines, the Holt and some of Dale Dietrich (sp?) designs come to mind. Jerry Howell has a neat V4, as does Bob Shores. You will put a lot of work into any multi cylinder engine, start with a well designed plan.