I think your chainwheel is 48T. Single bigger chainwheels are available, but not so common as double ones with 52/39 or 53/39, which are quite common on Ebay. Be aware that you get different length cranks (from the centre to the pedal) and sometimes the cranks are rotated by 45 deg, which is no problem if it comes with the matching left side crank. Double ones are OK, and should fit straight on. You can leave the chain on the big chainwheel, and if ever you have some difficult to climb hills, you can hook the chain onto the smaller chainwheel with a twig or whatever, and then hook it back when you want more speed.