Alloy gears are good, I agree, and an ATD can be mounted on one if you can find a gear - they seem to have got scarce recently. Got a couple from SRM a while ago, but they had none last time I looked.
Frankly, given that you'd need to change the cb end housing, probably/maybe replace the bearing while you're on, get a cable and associated bits (the routing of which might give you a headache around the carburettor float chamber depending on your exact fitment), and modify or replace the cam ring, I'd stick with the ATD, although like Klaus I run both types. (Brand new end housings, cb covers and camrings can be had - but you'd be looking at a good few beer vouchers on top of the cost of a straight refurb.)
The great advantage of the ATD is that the internal timing of the mag stays at its strongest at all times because the drive is what's being varied, not the relationship between the armature/magnet and when the points open. The disadvantage is that they can get a bit tired, the gears can strip (always in the dark when it's raining), and they can also be quite expensive to fix.