I would have a hard time eating a $440.00 camshaft. Crane, & other makers make so many cams that work good in boats. When I was running twin turbo motors I had to have special cams made. If I remember right I had one made last year from crowler & only paid $225.00 for it. It was a hyd. roller cam. My option is in a tipical marine motor that only 5500-6000 not worth sold roller. But then what do I know.
I would talk to him & ***** which I guess you alread have done.
I think you are going to have a hard time proving you did not authorize the work. If you do not pay he will want to keep your motor. If price is under $5000.00 you can do small claims court.