How did you determine it needed a full overhaul?
Ive done hundreds of these Kohler head gasket swaps and 99% of the time the bores look like brand new.
Did your technician tell you which gasket was blown? i had a 2005 GT5000 personally, it blew the right cylinder head gasket and smoked very heavily, like they all did. I left the motor on the frame, dropped the exhaust pipe on the offending head, removed the tins and shroud, carb, and loosened the intake bolts on the one side, and removed that head, put on the new gasket...worked like a champ.