Your Continental has a freewheel hub, not a modern freehub/cassette system. The largest freewheel sprocket I'm aware of is 38T on a SunTour "Alpine" 5-sprocket freewheel. These are not common anymore. 34T clusters are more common, some are still in production. Otherwise, a new rear wheel with a freehub/cassette system could get the gearing you want, but you'll likely need to spread the frame to fit such a wheel. Regardless of which route you choose, you'll want a new chain to go with a new cluster, and it is likely you'll need a new, longer-cage rear derailleur to handle the larger gear spread.