Speedy,
You can have a lot of fun with the Cox engine parts. I have 4 engines that I made using the piston & cylinder from them. Roger Schroeder had a construction series called the Simple Single that was published in Strictly IC magazine that used the piston, cylinder and glowplug from them. It was quite popular and easy to build. There were a few mistakes in the article, most notable was the wrong thread specification for the glowplug and cylinder to crankcase threads. He called out 44 TPI instead of the correct 40 TPI on the threads. Since the threads did not engage much, if you made the threads loose enough they would work.
E-bay still has lots of parts for Cox engines. Piston-cylinders typically go for about $4 to $5 for the pair if you buy 4 or more. A few years ago I was able to buy complete Cox "Sure Start" engines for about $5 on E-bay so I bought a small stock. Used most of them up, but think I still have 3 or 4.
A V4 would be a little of a challenge to do in 2 stroke because the separation of the crankcase areas to get fuel transfer, but an inline 4 would not be terrible. I have thought about an inline twin to use the Cox cylinders and doing it in the same manner that I am using on the Lobo Twin that I am currently building. But, I have other things that I think I would rather work on for the next project after the Lobo.
Gail in NM