You are right about the details; it’s a very simple mixer using all descrete components and because of that, it’s likely to sound nicely warm owing to the 2nd order distortion (It’s sad to call it ‘distortion’ because it’s actually an enhancement!)
I’m sorry that I don’t have any circuit details of the AD62, I designed it a very long time ago, but it should not be too difficult to trace the circuits.
It would be worthwhile to use a switch-mode power supply although you will have to add some smoothing. A 2,200uF cap with a series 1N4006 diode driving it should do the trick.
As for transformers, my advice would be to try them and see what works.