A switcharoo for a ZA angine is easy and cheap. You'd be jumping from a 165cu.in. 4" stroke to a 206cu.in. 5" stroke just by bolting it in. The power and size are limited using the sideways flathead engine, but a Z at least has a center main. By boring, porting, cam, and such on the stock appearing ZA engine, the 50-60hp range is possible. There is a stroker in my area that runs somewhere around 70hp. This style engine breathes rather poorly due to the many bends the air has to take to enter the combustion chamber.
The later 206ohv (or almost identical 220ohv) will bolt in with very little modification (coolant hoses). Check your club's rules. The 220 LP gas power unit was rated 66h.p. at 1900rpm. It would take quite the ZA engine to get close to that.
The weak point in a high horsepower R is the tranny. That could be remedied somewhat from what I've seen. I'd pull an R in 3000/3250/3500#. There was a local R that split its tranny in 4500# deadweight.
The largest small block MM to my knowledge is 4.44" X 6.5" 402 cubes in a ZT. In talking to the owner, 445 cubes would be the hairy maximum. A local 335 ran 347 cubes for years in a 335.
But just for low budget fun's sake, 206 ZA engines are a cheap power upgrade, fun and easy to tinker with, and have some neat tricks to making them run well. I have many of those parts laying around as well as a complete 206 engines of both ZA F-head and 445 ohv style. Good luck with your project.