Its highly unlikely that anyone ever did a mod like this unless the amp was gutting for its chassis and power supply.
Include to the the complete chassis, transformer/s, power supply, the heatsinks and input stage to that, which is around 80% of the amp.
What’s left is the driver/output stage and bias mods. So yes I see it as doable, as he said he’s doing, but I think it would be limited to T03 case substitutions, otherwise would get messy.
And yes two different amps one Mosfet the other BJT using the same 80% above components.