I don't think there's a simple plug and play replacement for what you're asking (JJ's post confirms this).
Before you run out and buy an LPS, what about this?
If you can build your own adapter, you could make a cable (female to male) with pins 3, 1 or 2 (or both 1 and 2 for this cable). This will give you 5v only from the Mean Well supply. Basically, your not passing ±12vdc to the unit.
If you don't have the means to build your own cable, you could contact a company like
https://www.redco.com/ and have them build you a 1 foot cable based on the above. Or you could buy a ready made cable, cut it in half and manually twist wires 3, 1 or 2 (or both 1 and 2) back together, assuming you can figure out which wires belong to which pins (a multimeter would be helpful for this). Make sure you terminate the wires for pins 4 and 5 from the Mean Well supply (you don't want to short anything out with loose wires).
This will tell you if you only need 5v to power the unit. If it doesn't work, just stick with the Mean Well supply. Otherwise, you're going to pay through the nose for something custom, and it probably won't be worth the cost.
Assuming it works, you could purchase a LPS (5v, =>3A, >30VA). However, you would still need a different custom cable that would have a DC connector on one end and 5pin Midi on the other (using pin 3, and pin 1 or 2 only). This assumes you don't want to change the 5pin din connector on the unit to a DC connector.
If you call Tech support, and they tell you the ±12vdc is needed, make sure you ask them why. You might get someone who thinks it's easier to just say ±12vdc is needed, rather than look up the schematics to determine if/where it's used.
Edit: Actually, I forgot to mention, the very first thing I would do is open the unit up and see if the wires to pins 4 and 5 could be removed easily. If they can, than you don't need to the first cable I mentioned, you could just use the existing cable that comes with the Mean Well supply.