The rules are totally unchanged from the last time - 98 RON fuel, race intakes and no cats.
First of all, engine size is very important here; you want to make an engine large enough that it can make the power you need without running big cam settings or very high RPMs (which means you'd need a larger, heavier and more expensive reduction gearbox), but not so large that the engine is too heavy and (for future multi-engine challenges) with a needlessly large frontal area, which will affect the drag that the engine produces. Cost is very important too - if you are running a large displacement, low-revving engine, you can probably do away with a lot of the whiz-bang parts and fancy valvetrains...as an example, the last two challenges were dominated by pushrod engines, often with cast parts.
Apart from that, big, slow-turning propellers are the most efficient, so try to strive for that. This is in contrast to the previous two challenges where the blocking effect of the fuselage (for single-engine aircraft) or the engine nacelle (for multis) is now modeled. Also, even a very small flat-rating (where the engine is capable of producing more power than the propeller and gearbox can handle) can have huge performance benefits.