Thanks guys! I don't actually have an engine off of another kit, so I am planning a closed engine compartment, done up to look like the front end of those old Bentley racers from the 20's. So it will be a sort of box with a hatch you can open to work on the engine. Except of course there won't be an engine inside... maybe a drawing of one. I might leave the hatch open and just make half an engine so you can look into it. And then there will be exhaust pipes coming out of the engine box in a V shape and I want to include some rivets, something I haven't done on a kit yet.
As far as the underside goes, I am not planning too much. I still have to build the chassis out of styrene tube and because there are no exhaust pipes there will just be some electrical wires coming out of the cockpit. I have lots of parts though, I'll probably just detail it up with what looks good.