How to fix a lawn mower motor?

I have this old lawn mower(push mower) a friend gave me, and i had this plan of turning it into a go kart, but after cleaning up the motor i noticed the engine was locked up; so i sprayed down the inside with WD-40 and got it unlocked so i could pull the pull string. well when i pull the string it sounds like its trying to turn over but cant. it hardly gives the sputting sound like most lawn mowers do when pulling the string, and it gives this sound like air is coming out. How can i fix it to get it running?