I had thought about an underlay but the only problem is the black boarders. I really didn't want to lose the other original graphics though and never got around to experimenting with paint. What I was thinking is if you taped off the boarders from the top side then you could scrape off the paint on the bottom. Then just put tape on the bottom following your tape from the topside and paint away.
Glad to hear you are still enjoying the game (at least I assume you are if you are sprucing it up a little).