Bell 2420 Power Transformer Needed

I need to replace a power transformer on a Bell 2420 amplifier, whose plate winding has shorted.

The primary is 117V
There are two secondaries:

320-0-320
and 6.3V

The challenge is the dimensions--it's low profile-- 3.4" W x 3.7" H x 3.85" D.

Does anyone have a parted out unit whose transformer you can sell me? Failing that, any suggested after market retrofit that works?