I just went through a very similar situation. I actually ended up with two 7-speed cassettes from members here. One, a NIB 13-23 (the gearing I already had) and a lightly used 13-26 (the gearing I was hoping for). I wanted the bigger sprocket for hills so put on the 13-26, bought a new chain and am as happy as a clam. I seriously considered the readily available HG 13-30 or 13-34 cassettes, but went with vintage UG for the sake of vintage.
The absolute cheapest for you would be to simply clean it up, get an HG cassette, file it down, get a new chain and go with it. I think you could possibly play with the derailleur angle adjustment to get it to work with a larger sprocket and cut the cost of a replacement. Sheldon also
explains how to do this. Park also has a
decent rundown with pictures and diagrams.
MD