Assess the old mower you are considering repairing. Fixing a mower deck on a unit with a seized, irreparable engine is unproductive. Units worth fixing are those with cast iron gear transaxles and front wheel I-beams, grease fittings on jackshafts, wheel bearings, and mower deck pulley bearings, and lugged rear-drive wheels. Higher-end engines have spin-on oil filters and fuel filters, whereas low-end units are almost entirely stamped steel with minimal wheels, a maze of thin rods and levers, and unserviceable bearings. The cost of repair may be the same for either type of unit, but you may get an entirely different result---including a mower that still doesn't work correctly.
Estimate the required repairs and organize them by section. Major mower sections are engine and electrical, transmission/drive axle/brake, body and frame, mower deck, wheels and steering, and driver controls. Prepare a required parts list and obtain pricing. Re-evaluate the repair project on the basis of investment and decide whether to move on to the next step.
Procure parts and plan your work by section, starting with the engine first. Change the oil and filter, install tune-up parts, and check the starting and electrical system (including the battery) to determine whether the engine will run before you proceed.
Clean the engine thoroughly with degreaser and rinse. Dry it carefully and make sure it starts properly. Clean all electrical contacts, battery terminals, and spray on electrical system protectant.
Check and repair the transmission/drive axle as necessary. Change its oil or fluid per manufacturer's specifications. Install a new drive belt and adjust all drive train linkages. Start the engine and verify that the drive wheels turn properly.
Inspect the body and frame, verifying that there is no fatigue failure or sharp edges that could injure the user. Adjust the seat-occupied safety switch so the mower will stop if the driver falls off.
Tune the mower deck. Replace any worn-out blade spindle bearings and grease good bearings that have fittings. Use a bench grinder to sharpen and balance the blades; replace any that are worn or damaged. Ensure that the blades have the necessary friction washers and lock nuts to hold them securely. Replace mower deck belts and adjust tension per specifications. Clean the deck thoroughly and lubricate all height adjustment linkages.
Repack wheel bearings and replace tires that are cracked or worn. Inflate the tires to the proper pressure and reinstall them, using new C-clips to secure the wheels.
Check and adjust steering wheel, throttle cable, shift cable, brake linkages, blade control levers, and other driver controls. Lubricate the cables and control pivots.
Test the mower on level ground. Inspect all repairs and fasteners, then shift to neutral with blades disengaged and test-drive the mower, making sure it starts and stops correctly. Confirm that the parking brake works. You're now ready to mow with confidence.