Farnel sell a product called Jizer (we pronouce Gizzer of course) which is a great engine cleaner, it also had a rust protecting agent in it. This used with a steam cleaner seems to work very well. Then rub over it with some sand paper and primer with normal red oxide primer. Personally I think glossy finishes look a bit odd on engine blocks and I like the finish aluminium exhaust paint gives. That's how we do it barn tuning style round here anyway LOL