I've never come across a situation like this, but I can say for certain -- do not scratch. Marquee artwork is typically far more delicate than sideart.
I might try some of the lightest stripping chemical I could find, but I would wait a bit to see if anybody else chimes in with a better idea. Even touching up a clean marquee is difficult. Cleaning up the mes you inherited may not be possible.
For sure, go real slow.
Cheers.