Have you tried the trick of using a piece of typing paper to mesh the gears? You put the piece of paper between the clutch bell and spur. Snug the motor down. Then pull the paper out and make sure it looks wrinkled like an acordian with no tears or rips. Then turn the spur over a couple times to make sure it's not binding. See if that helps. Not sure what else to tell you to look at. Maybe check the front bearing behind the flywheel and make sure it's not wore out causing the clutch bell to move around on you. There is also a bushing or bearing in the clutch bell that could be wore out. Also, see if the clutch bell looks discolored like it is overheating and causing the gears to melt.