What some of the shops now are doing is replacing the guts of the amplifiers with something more modern. There is also a place that is replacing the radio guts with modern guts, the idea being to keep the front looking exactly the same but have nothing old inside. Buttons, knobs, dial scales have become expensive. Buttons and Knobs are the only parts I don't have an abundance of. I have LOTS of radios, probably 30 - 40 in various states of repair. One thing that would be really useful to learn to do is replace the belt (or motor?) on the Mexico cassette players and the Becker under dash cassette players.
My 2 cents.