The best I have found are actually two products made by Aurora in Canada. They are "Boat Scrub" and "Speed Clean".
The Boat Scrub is the primary cleaner and it contains some sort of bleaching agent. This does a very nice job on exhaust stains. For some cleaning jobs the company recommends mixing some Speed Clean with the Boat Scrub, but I can't remember if that is required for exhaust stains.
Aurora makes a variety of cleaners for all purposes, and I have only tried the Speed Clean and Boat Scrub. They are VERY hard to find, and never at a discount from list price. If you find them they will be about $17-19 per pint. And when you do find a discount, the shipping eats you up.
I wish I had a better recommendation - I would love to find a comparable product that is easily available and not so pricey. Perhaps others on this board will have a solution.
I have no affiliation or interest with Aurora. Check the link below and see if there is a dealer near you.
http://www.auroramarine.com/