Try a blow dryer, what you need is heat not solvents. Pick a corner and pull it back over the top of itself while applying heat to the gummy stuff. Expect to have "ghosting" of the fiberglass from the old decals because of the color change.
You may need to do a final cleanup of the decal gum. Acetone doesn't hurt fiberglass and will remove the last bit of sticky. Most decals are of a plastic-type material and acetone won't penetrate the surface of the decal...that's why you don't use it to remove the decal itself.