Hi bobmarlex, welcome to the forum and the string section.
The G-mark 2 amps have a great reputation for their clean sounds
I recommend a gentle restoration/refurbishing repairing only what really needs to be done to get the sound back.
You'll need a contact cleaner spray to get rid of the crackles of your pots.
Here's a guide to clean pots: viewtopic.php?f=123&t=9039
PLEASE REMEMBER to let ALL the spray fluid evaporate before plugging in and testing,
because the liquid is highly flammable.
More G-50 II including schematics: viewtopic.php?f=134&t=7006#p34155
To shine up the tolex on the cabinet use a vinyl restorer spray (silicone).
BTW. I'm not sure that the speaker in your cabinet is original, but if it sounds good just leave it in.
Original speakers can be found second hand online from G-50 112 II and G-100 212 II .