The only reason why I'm mentioning displacement is because it was a big part of identifying cars by class in the classic game "Sega GT." I've bought some cars in the game, even making my own Carrozzeria machines. Sometimes when I tried to enter cars into races, I can't because the displacement is low. And if I have a Carrozzeria car, I will have to sell it, and build a brand new car with the correct engine specifications for me to enter. When it came to the engine in Sega GT classic in Carrozzeria, you chose displacement, engine type, and aspiration. No money is affected by changing displacement. For you Zoom-Zoom! fans, you can even select rotary-powered engines. After all, rotary engines are great engines too. In fact, these are all the engine types I can remember you being able to choose: 3-Rotor*, Inline-4, Inline-6, Boxer-4*, Boxer-6, V-Type 6, V-Type 8, V-Type 10, and V-Type 12 (* = not sure if such things existed in Sega GT). So if you want your Mazda to make more HP from a rotary engine, or if you want to sample a rotary for your racing car, you probably can.
I think we've got a little of the European perspective on this. How about some of you Japanese, Australian, and especially American types on making race engines? If you play PS2's "Ridge Racer V," you can race with a normal engine for any of the six available cars. As you progress into the Extra class cars, you receive racing-spec engines. As you make it to the fourth and final championship, you get an Oval engine for the Airport Oval course. Also, how would you feel about purchasing crate motors, buying specialized engines (for example, a Judd V10), and things of that nature?