RE: What engine makes the lightest conversion?
You can get a small echo trimmer, the 21.3 cc model and get a CH ignition system. This is exactly what the FPE 1.3 ci engine is. The FPE 1.3 sells for $440.00 and you can do the echo conversion and have the same thing for about $300.00 if you buy everything new. If you want to get off cheaper find an 18cc or 21cc weedeater featherlight and save about $75.00. It wouldn't have as much power as the Echo, but pretty close. It would work fine on a cub.