[Christ]

So taking this beyond just the piston, bore, and rings, a few things you can do to make the engine itself more efficient are:

Highly polished friction surfaces (bearings). If you want to try something special, use copper sheet and make your own bearings of the correct clearances. Copper sheds heat well, and for a low-speed engine, friction is already minimal, and since most oils have zinc additives, they will protect the copper from abrasion.

Another thing you can do is swap the rod out for one of a large enough size that you could install roller bearings instead of standard bearings. This might also reduce friction.

The area along the top of the piston, between the crown and the top ring land, is considered a "gap" area. Mixture will gather there and not be ignited properly through the burn sequence. If you have the skills, try to design a top ring that will fill that groove, rather than allowing up to 30% of the overall mixture to occupy that space at TDC.

Consider testing rings made of high-temp nylons instead of metals. Nylon is self lubricating, but wears quickly.

Don't forget the timing gears and cam friction surfaces, as well. Every bit of friction needs to be looked at as a package to get maximum friction reduction.

Helical cut gears reduce noise, but increase friction a good bit. Side or spur cut gears will be noisier, but reduce friction. Side cut gears with a good polish and proper lubrication will give the best performance as far as friction goes. Less teeth per area will also reduce friction, but that can only be taken so far and maintain proper operation.