Finding a replacement voltage regulator... what do the codes mean?
Your first item (the MC78T12) would be a suitable replacement for the 78M12.

UA indicates the manufacturer (Fairchild, I think).  Many manufactures make this series of regulator, and all makes should be interchangeable.

78 indicates a series of linear voltage regulators

M is low current (500 mA), so the 78T12 is a bit of overkill...

12 is the output voltage

UC is the package type (TO-220) (different makers use different package codes, just to confuse the user  :-) )

Your second link is to a switching regulator - a quite different type of beast.