A: Mika Covered the Liniar Supply issue (while they can be used when you get to the high loading they are more likely to cause faults) if it were a regulated Linear supply it would work but I don’t know of any that are regulated on the high voltage bus, Generally they are on the 12v or 5v busses only as micro controllers are run off those.
The SDR-480 is probably the most suited for the job but if you had chosen the 36v MSP-600 then you are running at a different speed and the SDR-480 would result in.
Options you have:
#1 - SDR-480-48 (720w peak output 48v) - there are certain situations with this unit when the Boost drains and capacity is still needed where the system can fault due to current limiting)
#2 - Add a second MSP-600-36 - These PSUs are able to current share which means you can hook two of them together and they will work as a single 1200w unit.
#3 - RSP-750-48 - should be more than enough power on tap at any given time.
You could also try a 24V unit but that would slow the servo more and would change some output character… It looks like the ONLY way to stay in the 36V range would be the Parallel MSP.
There is a 36V SDR oddity on eBay but I am not sur if it is something custom or Old as Meanwell does not list that unit anywhere SDR-480-36GV. I have never contacted the seller for a data sheet but if it is an SDR it would have a Wattage Reserve Boost.
B: I believe you can still get these from Mige Direct.
C: Cut the onboard resistor and then attach to the board pinout for the external… The Newer boards have a jumper where the external would attach… NOTE: - if you use an aluminum encased Power Resistor they are rated at HALF wattage when used without a heatsink… I have a 50w external attached to mine and I think it was the first with an external because I think My SimuCUBE was one of the first to kill itself. (before many protections were put in place and before the Wattage upgrade to the boards) + I was running at the limits with the Large Mige.
You can Just increase your wattage without cutting the onboard resistor and adding an additional external Resistor of the same rating as the Onboard resistors… as long as the resistance is the same the wattage will add to the others.
In all reality though if you have the 20W SimuCUBE Board you probably will never run into issue with the protections in place.