fast_mag,

unfortunatelly, our little airfield, despite our care, show a roughly grass almost all the time, and my Hanno's projects remain in his first stages because it couldn't run well in this surface (low ground clearance for the propeller, and bad field for a wing's based landing gear).

Despite this, in my study of the plane, I have collected the impression that the Magic is a ballistic aerobatic plane, and thus it will need speed.

With a short stroke engine, I think that there is no problem. Also, best with pump if the system works with more reliability.
Despite this, a long stroke give you the chance of use for a more long diameter propeller, and thus change some speed for traction.
It will work better for the aerobatics, but it gives to you some problems with the big diameter of the propeller, the larger landing gear needed and the greater space required in order to retract it.

If I have your luck in having various engines, I will try first the best and reliable engine system for a 12x6-8 and see it.
Then, don't worry about cooling openings. A full open in the under part of the cowl is almost required and wellcome in the warm days. A sloping deflector in the low part of the firewall also helps, as dave show in the pictures.

Finally, my congratulations to you for to be an scratch builder, and moreover for to build the Magic; good luck with it!.