We need a new procedural engine, with a torque curve and a proper sound, that can link to propellers or to a gearbox which can be linked to wheels.
This engine should have a changeable number of cylinders, displacement and configuration (ie, radial, flat, V, inline) and also have options like air or liquid cooled, carburettor or injectors, petrol, avgas or diesel etc.

The quality of engines in the base game is truely miserable at the moment, to the point that they don't even spin at their indicated RPM or make the right tone noise. The advantage of SP over SR2 is it is much more intuitive to make older things, and I think a procedural piston engine will suit the game well.