Not all Sachs 4.2 are alike. They all use the same cyl, piston and crank, but the crankcase, prophub and ign systems can be very different. I just set up a Sachs 4.2 for a friend that was indeed not much heavier than a stock G62. But remember, Tom's prototype flew with a G-62 converted to electronic ign, which is a bit lighter up front while shifting some weight behind the firewall (module + battery).

I am sure you can do it, but a Sach 4.2 is really happier in a 30%+ plane with around 90" wing span, like the Lanier Edge 540.