If it's just the mast, I got mine from Mid-America and replaced it 2 weeks ago. I didn't remove the motor because mine seemed like it was just jumping a couple of teeth. 10 minute job to replace just the mast.
But as you will find with most of the posts, if the plastic is broken or stripped you will need to drop the motor to clean it out just like John described.
Note: I got a wrench with my replacement mast which is suppose to help take off a large nut holding the antenna on after you take off the rubber grommet. I did not have a large nut on mine but a small one that I had to use a needle-nose pliers on. If you have a large nut and need the wrench, let me know and I'll send you the one I have.
Good luck.
Ron
"Baldie88"