You can use a Sunrace 7s 13-25t freewheel with that derailer and shifter, but will need to use 9s chain for it to index smoothly, since the spacing on Suntour/Sunrace freewheels is a little different than Campy intended.

There is a "B" tension gizmo inside of the top knuckle of the rear derailer. Removing the derailer exposes the toothed stop that you will want to move forward to make the derailer move rearward for the larger freewheel.

I doubt that a larger freewheel can be found to work with this derailer, but I have to admit that I didn't try one. Possibly a 28t would work, I don't know, but a longer chain is needed, one link longer for every two extra teeth on the largest cog.

Don't forget to put some light grease inside the indexing shifter, down inside where the two indexing springs and notched indexing ring are are. These shifters work quite well if kept lubed, and these Synchro systems work better than they ever did back in the day using today's chains. I used Shimano 9s chain btw, and the shifting across the freewheel range is crisp and smooth.
It will also help to use a new, lined piece of cable housing at the rear derailer. Properly-fitting ferrules are important, so don't lose the originals and use modern Brake cable housing if that is what fits your derailer and stepped ferrule best. Keep this housing as short as practical, given the new positioning of the derailer after the bigger freewheel goes on. I so often see overly-long loops of housing out back that do nothing for indexing accuracy. Double-sprung derailers need added length to accommodate the swing of the derailer body, but yours doesn't.

Here's what a shortest-practical housing looks like, the first two shown are even accommodating double-sprung pivots (yours is fixed).
The third photo shows a derailer just like yours, together with a 13-25t Sunrace freewheel. The cable housing shown is a bit longer than necessary but works fine.