If it ain't broke, don't fix it!
besides the mains fuse, there are two smaller fuses for the + and - voltage supplies. They are probably fines since the amp powers up.
The wires may be brittle from age, and you may have broke one or more while working on it.
If one speaker works, check the wiring to the other.
Try swapping the speaker wires around to see if the speaker is okay, or if one of the outputs is bad.
check and clean the fx loop jacks, and all the output and switching jacks on the back panel.
You may want to replace all the input jacks if they are falling apart.
Write down where all the wires go, take your time, be careful and don't get frustrated.
I worked on one last week that had a couple of intermittant connections and loose wires, but these things are built well and usually work pretty good with some maintenance and TLC.