If they are the originals then the smallest you can get away with is probably the same as what you have. Winches are expensive and builders do not oversize them. What I can suggest to save yourself some money is get a used one from Bacon Sails
over on 116 Legion Avenue. If you aren't looking for a self tailing model, they have a ton of them at reasonable prices. You can view their used winch selection on-line using the hyperlink above or simply call them at (410) 263-4880. In any event, you will need to get the old one off the boat. Do that before you go and take it with you. They'll help you pick out the best replacement.