I wouldn't drill the sump for a mag 12 for 3 reasons personally (and I have tried this myself and changed my mind about doing it quite quickly

#1) The mag 12 is kind of noisy when it is run extrernally. This may or may not be an issue for you depending on where it is and what your tolerance is.

#2) The mag 12 is an internally cooled pump. The pump heat is getting transfered to the water running through the pump regardless of whether or not it is run internally or externally. If you put your hand on an externally running mag 12 it will be cool to the touch (about the same temperature as your water as a matter of fact.)

#3) If you take the pump apart you'll see that the only thing holding the water in is a single rubber gasket. This may or may not be a big problem, again depending on the location.

When I just put it in the sump and used it as an internal pump I didn't have any problems with it. It was a good pump. Anyway, good luck with your project! Ditto on using a standard bulkhead if you decide to go that route.