A .70 4-stroke wouldn't really be overpowering it. The difference between a typical .52 4-stroke and a typical .70 4-stroke is about the same as the difference between a .40 2-stroke and a .46 2-stroke. Saito is the online major manufacturer still producing a .62 4-stroke, with most of the other manufacturers producing either .52~.56 4-strokes and then .70~.82 4-strokes.

The YS Engines .63s 4-stroke is supercharged, and is more similar in power to a 4-stroke in the .80~.90 displacement range.

Good luck, and good shopping.