A few thoughts about the engine designer.

1: Cooling - I think i would be a nice touch to be able to choose cooling systems for your engine. It would probably effect price and MTTF and how hot you can run the engine. It would make the engine feel more real (less generic).

3: Timing belt : It would be nice to be able to choose between a timing belt, or chain or double chain- system. The chain system would be more costly to produce, but lower the service costs of the engine (and perhaps increase the MTTF a bit).

I also would like to see the ability to tweak the cooling system.

I was thinking there should be an option for a timing chain myself, so +1 on that.