[Arran]

In my opinion I wouldn't bother messing with it as it is, find another chassis and make a good OEM chassis out of the two. This 90 is almost as bad as that Philco 643 battery set you had. The replacement speaker is probably acceptable to reuse, and would actually be an improvement over the old pie pan type.
The trouble with extensive modifications, if not out in out butchery, is that you really have no baseline to start from since the people who do theses sorts of things don't usually leave a schematic. As you say you can't be sure that whoever did the work did know what they were doing, some of these sets look like there were someone's weekend project so who knows whether they ever finished it and got it working properly, if at all.
Regards
Arran