I agree, just get some Grade 5 bolts and nylock nuts to secure the clevis. If you want to get even more security, then put a regular nut on followed by a nylock nut to act as a jam nut.
Since I see a cotter pin holding the clevis pin, I would think this would be more than adequate to secure it, unless during mowing the cotter pin is being sheared off and the clevis pin is vibrating out of its hole.
Personally I wouldn't spend the bucks on an extended warranty if this is the only issue you are having with the tractor; nor would I trade it over a nuisance pin problem. Now if you are blowing head gaskets, having oil leaks that can't be repaired, gremlins in the electrical system, etc; then I'd suggest trading it. But you haven't mentioned anything of the kind, so I'm assuming the tractor performs like a champ in all other activities.
