What do you mean by "run backwards" ?

Do you mean swap the inlet for the exhaust, like on a touring car Vauxhall Cavalier XE engine but the engine rotates the same direction still, or do you mean the engine physically rotating in the opposite direction?
(ie like a ITR DC2 honda engine)

The two are very different animals, I can give you some pretty good pointers on either, but cant be bothered to type out for both when one wont be needed so please be a bit clearer about what you want to do.