Those are rivits or like rivits. I would try to fix it with some grade 8 machine bolts. The problems the clutch/belt side has to be smooth for the belt to ride on. You cannot have bolts sticking out. This is why the used the rivits. Not sure if you can find a person with a rivit gun that big. I sure don't.
If you do weld it make sure the brands or rivits are smooth on the belt side. That slack has to be someplace. See if you can weld just the rivits and not on the face part of the clutch. This way you could still grind the brands off. If you do weld the face. Just tac weld in a about 6 spots. No heavy welds. You may have to fix it someday.
Best wait for other help though man.