I'd have to check my books to be sure but I believe one problem your going to run into is that the poney engines don't turn anywhere near as fast as the other small engines your talking about. Then there is the matter of the poney having a true flywheel and clutch setup on it that the other engines won't have. Without that there is no way to stope the start pinion and engage it into the flywheel ring. Without being able to do that your talking a major expense to put in new flywheel rings on a regular basis.
That said, I do agree that pretty much anything is possible. Unfortunately in instances such as this one, I don't believe the cost and time spent trying to 'reinvent the mousetrap' is worth it.....especially given the work necessary to pull the engine to replace a broken/damaged ring gear if something doesn't go exactly right.