Right i think the solid axle thing should be possible,
Heres how, depending on where the hole for the pin is in relation to the bearing you could just drill through both the pulley and bearing.
Tap the hole and find a grub screw that will sit flush or just below where the belt sits. make the end of the grub screw chamfered into a point and hopefully it will sit in the pin hole.
This in theory, should lock all 3 parts together, and if done properly should stand up to a modified motor.....
I do not have the car at the minute so i could be totally wrong.....