I've also been looking for a Mage conversion, as I love Mage and its various iterations. I really like the logic of its magic spheres, and the narrative nature of the game. I have yet to see a magic system as deep and self-consistent.
However, I have always been unsatisfied with the WoD dice system. For one thing, the checks get a bit crunchy and situational. Also, for high levels of difficulty, the statistics don't scale properly. In the extreme case of a difficulty 10 check, you can roll 100 dice and have equal chance of botch and success.
Genesys looks like a very natural engine for the game. I think it addresses the narrative intent of Mage much better. Also, there are a lot of similarities between the two systems:
1) The storyteller system uses character stats between 1-5 with a human average of 2. This is the same for genesys.
2) The storyteller system uses simple difficulty checks and counts degrees of success. Opposed checks where appropriate. Same for Genesys.
3) The storyteller system uses degrees of success. Genesys does this, plus the advantage/threat axis for flavor.
My initial thought is to take the Mage character sheet and experience system, and use it directly with the genesys system rules. Most things map directly, at least statwise. Looking over the Mage magic rules, my quick thoughts are:
Magic Stat - Arete, and I'm not sure what to pair it with. Could use intelligence, but that elevates that stat above all others. Possibly don't pair it at all, and upgrade dice with quintescence. I think foci and rituals should add boost die.
Magic difficulties - Coincidental: Highest Sphere; Vulgar: Either difficulty +1 or upgrade the challenge; Vulger with witnesses: Either difficulty +2 or upgrade the challenge twice. Increasing the difficulty fits closer to the M20 rules, but I like adding the challenge dice. This brings in despairs, for which likely effect is paradox backlash.
An obvious use for threats is to generate paradox. I would think to use paradox in place of strain for magic, so coincidental spells add paradox equal to the number of threats, vulgar adds 1 plus threats, and vulgar with witnesses adds two plus 2 times threats. This may be too heavy-handed with the paradox -- so a bit of testing is in order. I do like the idea of generating paradox even on successful casts.
Advantages can be used creatively. If nothing else, they can bleed off paradox, at a rate of one per advantage. This way vulgar magic is somewhat risky, and vulgar with witnesses is always dangerous.
I'm curious to hear thoughts on this, but I suspect it will be an easy conversion. Easier than running Mage in its own dice system, really.