John Deere model L120 has a CV40 20 HP Briggs and Stratton engine with a Hydrostatic Transmission made from 2003 to 2005 with a 48 in. cut. The very first thing I would check is it the drive belt or something else very simple? If it is the transmission you could be looking up to a 1000 dollars or better with new parts and labor. Is the mower worth fixing is something you will know better then anyone on this forum. You wouldn't replace that mower for a 1000 dollars so just what is the rest of the mower like to spend that type of money?