also drain the tank and check for rust. this bike has been sitting more then riding over the years. replace all fluids, clutch, brake, cooling, rear end grease and lube. if original tires, I would change them.
I get my keys made at Lanes Yamaha. The owners wife is amazing, she can look at a key and Knows what key code to use. Then punches in the code and out comes a new key. apparently there are only so many variations to Yamaha keys.
Other then an occasional Bee in the helmet, Robin loves the new pillow tops I installed on the 99. She seem to spend a lot of time sleeping back there....must be comfortable.
In the manual I have it shows a ground lead attached to the radio casing, pg 8-54. You did say you have nothing on the display right? no power at all? If that's the case, then it's not the mute. All though I don't think it has a mute mode. Don't have my bike right now to check it out, it's in the shop. The CB is under the trunk. Worth checking the wires under the seats.
Did you check the ground connection? If I recall you said the po said it just quit working will out riding. Maybe a loose or dirty ground or broken wire? Did you check all the connections for corrosion?
look for pin hole leaks in the rear Y pipes, these pipes seem to rust out faster then the front pipes. I believe they are made out of a different material.
We went there last year. Didn't intend to, just kinda ended up there while looking for a hotel. We took the shop tour, very impressive. Got to meet the shop manager, can't remember his name now.