not sure if they will take it back if it has been welded tho it might have voided the warranty
If the guy sold it to you without telling you about this - as a like new mower with no problems, he would be getting his mower back if it were me. I've taken a couple things back that I've bought and it is important that you remember - your not asking him your telling him he is getting his mower back and you are getting your money - PERIOD. You go asking him and he will know you are a wuss.
Other option take it to a real fabricator and see what they can do - then take the guy to court to pay the bill to fix it - this option is a long shot to get the money though.