Hmmm, I tried that with Omni and a month later it was still tacky !!

I use PPG Omni single stage for all the parts you describe - engine, trans, suspension, etc.. I prime with epoxy primer. I do all this stuff with either of the two cheapo Harbor Freight guns. Cleanup is the worst part.

Parts that are not prone to seeing any damage are done with rattle cans, seat brackets, pedal mechanisms, etc.