What kind of noise is it making? The slipping could be the centrifugal clutch, but usually there isn't noise associated with that. So what you are experiencing that feels like slipping could be a lack of oil pressure because that sometimes has noise associated with it. Or it could be a more internal hondamatic issue.
Honda doesn't sell parts for the hondamatic and says they aren't rebuildable, but they actually are pretty simple inside and there is a guy here in MN that buys up bad ones and sells them rebuilt, better than new (better bearing inside). That's for the Rubicon hondamatic but I'd guess he could do the Rancher 400 too. I think he gets $550 plus your core for a rebuilt one.
You just need to decide if "the juice is worth the squeeze" or not.