Help identifying replacement regulator
I would assume that's a version of LM78M12, which is a 12V regulator. Have a look at the datasheet for the pinout of your package (http://www.ti.com/lit/ds/symlink/lm341.pdf - I know it says LM341 in the link and LM78M05 in the PDF, but the layout is the same). 

Once you identify which one you have (TO-220 or TO-39 package), check the pinout and measure pins 2 and 3 with a DMM. If our assumption was correct, it should read 12V. From what I gather, these were 500mA regulators, so something like a regular LM7812 should work fine.