Just replace the monitor with a good one. It's one of the most standard pieces on that machine, and one of the most likely to go bad. It won't hurt the value to replace it with just about any other decent standard res monitor out there.
As for the sideart, the repro vinyl is crazy expensive. If you go that route you'll probably be spending at least $300 just for full artwork. Using the repro artwork might hurt the value in the eyes of a true collector, but it will make little or no difference to the average hobbyist or Joe Shmo that wants a pretty Ms Pacman in his house.
If the paint on your cab is so bad that you can't make out an image anymore, I'd go with the repro sideart. Otherwise, if you still have an image (even if it's bad), then you have a guide to paint with. It would be like one of those old paint-by-numbers thingers (in theory).