I have no clue about Hawkmoon, but I've played a good bit of Ars Magica. It appears to me that the system was designed as follows:
1) Observe that all current magic systems largely suck.
2) Sit down and put some real effort into producing a detailed, flexible and atmospheric magic system, combining some of the best mechanics ever with some excellent flavour text.
3) Produce a detailed list of spells and a flexible and easily understood system for inventing your own, and an excellent system for producing magic items.
4) Prepare to publish!
5) Remember at the last minute that roleplaying systems do need systems other than the magic system such as task resolution, combat and so on. Shoot. Bang such systems out in an hour such that they look vaguely credible at first glance.
6) Publish!
As such, I suspect that you would be better advised importing the magic system into the Mongoose system, which should be a fairly easy conversion job, than using the system straight...
