I have a Sig Senior Kadet with a O.S. .61 FX and I'm looking to change the engine to something that uses less fuel but still plenty of power.

I was looking at a Davis Diesel conversion but that requires a special mix diesel right?

Are there any Gas engines small enough that run pump gas that are not to heavy for a Senior Kadet?

This plane needs plenty of power because it has is own GPS microcontroller self guided flight system. (School Project) But we are looking to extend its flight time, i.e. use less fuel. Any Suggestions?