I had a Magic Extra, since retired, and originally flew it with a Magnum 46 (the motor was on sale) and then with a Magnum 52.

The 46 is not a bad engine and will certainly fly the plane, but the 52 gave it an extra bit of punch. Although it sounds like you're leaning towards the 52 anyway, and I say you go for it.

I second jship's recommendation for the APC 12.25x3.75 prop. It is a great fit for the 52 and this plane.

Now, in reference to OS vs Magnum... personally, I think the Magnum motors are great values for the money (especially their 4 strokes). However, the Magnum 52 is notorious, both on the forums and at my field, for bad midrange transition. The low-end needle is difficult to get just right, it is either too rich (blubbery transition) or too lean (dies) and sometimes the difference is 1/16th of a turn! After break-in, turn the low-end all the way in, then out 1 turn... lean from there. My low-end needled ended up about 7/8ths of a turn out. As a side note, the Magnum 46 did not seem to suffer from this problem.

Another option you may want to consider: A fellow at my field with a 52 replaced his Magnum stock carb with one from an OS 46FX and the transition was MUCH better with hardly no fiddling. This has also been reported by others on the forums. If it drives you nuts, this may be a solution.

The Magic Extra fuse tends to break easily right in front of the wing and behind the tank. You may want to try to reinforce that area, although my attempts were met with limited success. It is a great flying plane, and you'll enjoy it.