The original belly skin is 0.025". That's hard to come by at
Metal Supermarkets except by special order. I had several jobs on my Argosy and bought a few 4' x 8' sheets of 0.032" and made it work. I cut a piece of scrap into 3/4" squares with a hole for the rivet in the middle -- those were my washers.
The whole belly wrap is usually going to be thinned by corrosion but fairly easy to patch. Lap your patches like shingles so the inside can drain toward the rear. Remember to avoid sealing the belly skin margins -- it should breathe.