I am currently looking to buy a White LT1500 lawn tractor with a 15.5 go Briggs and Stratton engine. It the description of the ad it says it only goes in reverse. They are asking $275.
How lucky do you feel? Unless you are mechanically inclined, I would stay away from it. Could be a simple adjustment or a gearbox teardown. Final cost could be as much as $600, depending on who does the work, if it is even worth repairing.