have the specs for the transformer? There is usually a way to sub something else but it may require a little creativity. Different mounting holes, maybe it will have an extra winding, stuff like that. Hammond makes a bunch of them, so does Edcor, or you might find a good used or NOS transformer from any number of makers on ebay. Stancor, Triad, Merrit, lots of others made replacement transformers and they still turn up.
the exact replacement is the easy route though. I just put one of the Mercury ones in a solid state Sunn amp that is on my bench. Bolted right in and so far as I can tell its an exact match for the missing original, except not as ugly. The originals seemed to have been very poorly dipped in varnish and they are lumpy.