Uuuhhh...
Okay, the engine sounds don't really matter much because you can basically mod them to be whatever you want it to be. It's made like that for versatility and ease of use.
You can make ZR130 sound like a locomotive for all I care.
Engine swaps could only come as a visual model, but it's something that is not really visible ingame, and being in a low poly environment, we don't want to go too overboard with it, otherwise we break the low poly style (which is already broken with enough parts like wheels and spoilers). Moonbeam already features visual engine swaps, but who can tell if you can't open the hood? It's almost pointless in more usual and realistic scenarios.
But hey, it's in the game. Thank me later.