Who wouldn't want to put a 1.8L Vtec into a Fiat 500? that in my opinion is an awesome idea. Throw in an FTO engine or even an Evo engine (with 4WD) into a colt. A buddy of mine is currently replacing the 1.6l (non mivec) engine in his Mitsibushi Colt with a 2.0 V6 from an FTO, along with the suspension and brake hubs... basically the whole lot.
This stuff happens a lot in real life, and it would be awesome in game, would make for some pretty cool combinations. 700bhp 4WD Colt? 350bhp Vtec Fiat 500? (though that would be a mammoth task, different manufacturers) or a 2.2l Vtec prelude engine into a Civic. Upgrading a Subaru Impreza with a Cosworth block (as is common with tuners). Putting an RX7 engine into an MX-5.
It wouldn't be easy to impliment, but it would be pretty awesome in terms of customisation if they got it right.
The Fiat 500 with a Vtec engine is probably on the ambitious side, but it's probably not impossible. Stuff with matching manufacturer though, thats quite simple, in some cases its just the case of dropping in the engine.