Jim,
There will only be a few degrees difference in boiling temperature between 13 and 16 psi. As long as both 16 psi caps you've got are the reverse-flow type they should work if they seal on both the inner and outer rims of the tank. There are a couple of different heights from inner seal to outer, so you'll have to look carefully for the match. The cap must have sealing gaskets for both the upper and lower surface.
