RE: What engine to use
A .91-size engine, or even a new .75-size engine would be equivalent in power to most 1.20 four-stroke engines. Here are two suggestions:
OSMG0575 17400 .75AX AIR ENGINE US$339.99
OSMG0591 19021 .91 FX RINGED W/MUFFLER US$349.99
These parts can be ordered through any hobby retailer who sells O.S. Max engines and accessories.