The documentation I have describes the tape as high bias cobalt-modified, which rules out 456/457 or similar. Maxell XLII EE tape would fit the description, as would some thicker videotapes that are much like high bias cassette tape.
The problem is finding EE reel-to-reel tape in half-inch, and low grade, longer length (thinner) VHS or Betamax tape wouldn’t be suitable at the 7.5 ips speed of the MG-1212. I’m sure people have tried normal bias tape like 456 and it may seem to work to a degree, but you can bet things are whacked compared to the original specs of the machine.
The shells are similar to betamax but aren’t interchangeable. They where originally made for Akai Video cameras in the 70’s.