However, since the OP says he'd like to try a rear wheel with a cassette (meaning a freehub and not a freewheel) that will be very hard to find NEW to fit the 126/7 rear spacing. Either shop for a nice well-kept vintage set that uses the shorter spacing (can be done but will take some careful shopping and time) or spread the rear 3mm (not much of a spread) so you can select from all the new 130mm rear wheels with freehubs/cassettes that are all over the market. You can even use a 7S cassette with a spacer on the freehub body, or from 8S to 10S (or often 11S) without a spacer.If you search for Sheldon Brown's instructions for DIY rear spreading he makes it pretty easy. Or if you doubt your abilities take it somebody you can trust. Or just go with a freewheel, keep the frame as-spaced.