Rivets = basically you drill out the old ones and replace them with new ones as needed. The size of the drill bit will vary, especailly on a patch, depending on what the PO did to repair/patch the section you are talking about. (pics will help) normally a 1/8" - 5/32" is used, but if the PO overdrilled the holes or used a bigger rivet, yours could vary.
In the upper right blue bar of this page, you will see a "Search" button. Click it, type in a few key words and many similar threads will pop-up.
As far as identifying the original layout of your trailer, check this out:
There are several '63 O'landers represented there.
Good luck with your repairs..happy posting!