Measure your boat. Here is a simple guide to measurement e-sails.com/measuring/ Use your halyard to measure the luff and a tape to measure the basic foot from the stemhead to the mast. This gives you the basic dimensions for a 100% foretriangle. The leach measurement depends on the cut of the sail and the height of the clew. The original would have been around 130% so overlapping the mast by about 30% of the basic foot.
Buying used sails is usually a waste of money because rarely do people sell cruising sails that are any good. The boat's performance depends on the sails, particularly the foresail and the chances of finding a good used sail of the correct size and cut is slim. A basic new sail will be dramatically better than a used one. If you can't afford a custom made sail from a good UK loft consider the ones in the link which are the "budget" range from Kemps or look to the various Far East lofts which on a straightforward genoa will be a little cheaper. No danger of them not fitting as the lofts have the dimensions and specs of the original sails.
