I bought a used 2012 Beaver Tail surface drive. It seems as though the clutch is slipping when under load. Occasionally it grabs and I can get up to speed but more often, especially after running for a few minutes, it just won't grab. I'm considering replacing the clutch but can't find a replacement clutch online. It's a 38 tooth Gator Tail clutch and has 2 sets of numbers. SO-3215 and 689-0159. Any recommendations? Besides the clutch, what else could be the problem?

Sounds like you have a bad connection going to the clutch.
Make sure everything is good and tight from the switch to the clutch.