That is the same 4 speed engine as found on the puch monza and a lot of Spanish shifty cafe and cross style bikes. I have one on my puch mv50. Super engine. Same stud pattern as a maxi but you probably have to enlarge the cases to fit any larger than 50cc cylinders as these have a smaller hole than maxi cases. But I never owned a x50 so there could be subtle differences between this and a monza engine. Could even be a 2 or 3 speed.
Anyway cool bike. I'd leave the ratty awesomeness