As Grassdaddy said, find a motor from a used Ride on. You should hear it run B4 you part with your dollars. Listen for any rattles as you accelerate the motor from idle. Look for smoke from start up.
See if there is a chassis related reason for the Ride-on to be not-worth-fixing. Sell the wheels to some kid for a go kart! Then keep your eye on the oil level from now on.