[Arran]

Check the value of the resistors first, they may have drifted upward, but maybe not if they were 1 Watters to begin with, no sense replacing and replicating unless you have to. As mentioned the big power resistors used as a voltage divider and for tube bias (part #25) are a common failure point as well, but you may get lucky As for anything else, the model 20 is a fairly common early 20s set (at least the Baby Grand versions) so lots of documentation and experience to be found with restoring one, some people like Mike (Morzh) have owned and overhauled several of them.
Regards
Arran