Look for sills and floorpans filled with:
chicken wire
polyfilla
beaten out Coke cans
newspaper...
Don't do what a friend of mine did and cram a Metro Turbo engine under the bonnet of a Mini City. It's not big and it's not clever. Especially if you don't change the badges, seats, wheels, brakes....
However, it IS alot of fun - especially when you absolutely destroy a fully race prepped 1400 Clubman. I'd like to say that this little contest did not take place on the public road and threaten the safety of other innocent road users. I'd like to say that, but I can't....