you do worry me Scott.
Vaine , you must remember Scott is a motor head just like me. He competes big time and HP is his game. but sometimes its not all about the HP. As a motor that is probably one of the easiest to work on a convert to marine. it has lots of old articals showing how to watercool it, and the crank is solid with bearings at both ends. its also very compact and has its own clutch which is one of the reasons it was popular.
With that said these motors were around during the start of our great hobby. i have a box-load (40) of them sitting in storage brand new never run. they make great model boat engines and i have even seen a couple that were converted for planes. as long as your not looking to go Mach II. I have several old boats powered by watercooled versions of this Mac/Kiortz engine. they run all day long with no problems . they can run 20-30 mph in 48" hull depending on your set up. its not 50? but its still fun.
these pics are of the first boat i ever built in 1985. it has a mac engine and an AMPs drive on it. Still goin strong today.
good luck