[Arran]

You can test the windings of a transformer with a regular Ohm meter, it's won't tell you everything but it can tell you whether one leg of the high voltage winding has a different resistance then the other or whether a winding even has continuity. Most schematics give DC resistance measurements for the transformers. Generally speaking if a power transformer has failed catastrophically you can tell just by the smell, it smells like burned varnish or a burned out motor, with a burning tart smell thrown in. If you can see the core of the transformer it will also look burned. Normally the leads going into it will also look burned and there will be a puddle of tar bellow the transformer, not just a little residue but blobs. I don't think it's too difficult to open the metal bells on one of these, just undo the bolts going through it and it should spread apart with a little persuasion. The model 91 used 6.3 volt tubes I believe so it should be as hard to replace the transformer as an earlier set, it's also vertically mounted as most newer off the shelf replacement transformers are.
Regards
Arran