Here's one of the quickest frankensteined CG engines I've seen, this one has an adapter plate on the base for the jug to blot onto and I think I read somewhere he's also using a longer rod to make it work. Power is claimed to be around 14hp and by the looks of the video that seems pretty accurate... *
https://www.youtube.com/watch?v=HOK9DsKZ_KQ there's also a thread by a member named four easy that grafted an Athena top end to one of these bottom ends with similar results... it's definitely doable if you can find the right rod and make an adapter/spacer plate to make everything fit and work.
I also got a 70cc Polini jug with a 47.5mm bore that I may Frankenstein onto a CG bottom end once I get some time to play a little, the jug I got has some scratches but the bore is cast iron and pistons are available for up to a 47.8mm bore so it's definitely rebuild able but it takes a 12mm wrist pin meaning I would need to re bore a cg rod, aquire the rod for this setup, or press in some aluminum or silicon bronze bushings into the piston so it can accept a 10mm wrist pin. This one would also require some case boring but I might back able to do that on my lathe if I can make up a jig to hold the cases.
This is another low priority project for me so it could be several months before I even start on it and get all the parts needed to complete the project. What caught my eye with this jug is that it's very close to the same size as a cg jug so the finished project wouldn't look too different other than the lack of an intake port, and the very large round exhaust port. With a fred head or similar it would be hard to tell it wasn't the original cylinder. I will post pics of it in the morning.