Reversing the wires is not going to fry anything, but you might suffer some power loss since reverse is not going to give peak current. There are plenty of top brushed speedos you can pick up for next to peanuts now that everybody is going brushless. For instance last week I saw on american ebay a GM V12 going for 30$ or thereabouts. That would give you a lot more power than you can handle.
Team Greasy Weasel
The best upgrade to any car is some driver skill.