[Arran]

The original transformer was likely a horizontally mounted one, unfortunately most of Hammond's offerings are not so you would have to make a mounting plate to cover the hole in the chassis, or it will look like the grey one which was bucksheed in there. However AES sells replacement power transformers for guitar amplifiers which are horizontally mounted, so it's usually a question of picking out the one with the right voltages and current ratings, so seven 6.3 volt tubes, plus a 5 volt rectifier. So five 300 ma tubes, and two 700 ma tubes, which equals 2900 ma, or 2.9 amps for the tube heaters. It's a sort of silly design, they used a 6F5 and a 6H6 when they could have used a 6Q7/6R7, if they had used a 6Q7, and a 6F5 they could have eliminated the interstage transformer, this seems to have been a fad around 1937-38 to "tube stuff" chassis this way just to boost the tube count, but not really doing anything useful.
Regards
Arran