Converting Shadowrun Magic to GURPS

[ArchonShiva]

Will roll against drain: Mages maxing out Willpower is a given in SR, so I'm not sure eschewing this will help you maintain the flavour. However, I agree that it's probably not a good idea to simply roll Will to resist drain in this case.

Whatever you do, dissociate Will from IQ and base it on 10. It's a canon optional rule anyway, and it gets you all six Shadowrun attributes as separate entities.

If you really want to stick close to the SR feel, I wouldn't try to make magic look too much like GURPS - it's perfectly in theme that dice work differently when working with supernatural forces. You could keep most of the rolls as per SR and only convert effect to GURPS when the time comes to roll damage or resist. I've done the opposite (using GURPS skills inside Mekton vehicles) and it worked fine.

If you don't want to play outside GURPS, you could take a page from Technical Grappling and don't be afraid to shift magic from roll-under to the damage roll mechanic. Force gives you dice, which you roll and add to attain an effect. A force 4 spell gives you four dice to roll, with your Spellcasting skill adding +1 per die at IQ+2 or whatever. Then make a similar roll for Drain, and pay fatigue based on how much above your Will you rolled.

These aren't complete worked-out examples, but just possible avenues to making magic feel more like its SR self.

Here's one that doesn't conform to my experience of SR magic, but does fit your list of criteria:
Let mages buy cheap Energy Reserves with an accessibility limitation. Use a Will roll for drain, with MoS letting you use some ER and the rest coming from fatigue.

By the way, unlike CrakkerJakk, I think that LoS range for spells (and the absolute requirement to see your target) is actually pretty key to the whole thing.