Improving engine quality

[Kaldari]

Keeping it simple, improving an engine's quality could be done using skill and spare parts. Great for when you find something you like, but the engine is rubbish.

Adding things like cleaning/replacing air filters, carburettors and spark plugs, or changing the oil would likely be more complication than it's worth

[Czechoslovakia!]

Engines require the player to have the car key, or to have hot-wired the vehicle, in order to repair or obtain spare engine parts. Removing parts will reduce the engine durability to 0 even if no parts are received. An engine can be repaired to 100% but may require additional spare engine parts.

In short, this is already in the game, though you need the key to the car, and mechanics skills to be able to do so, in fact, I find it quite handy to do this, dumping cars where I don't need them, and grabbing all the parts I can out of them for the cars I want to keep running.