Did it run after the rebuild or was it stored right away? If it hasn't run, pull the pistons and hone the rust out. Reassemble with the existing parts and use it. Might not be perfect, but it will probably be okay. Be sure and check the valve faces and seats, too.
Oops, I see where you said it was stuck. The rings are probably toast. Ironic that the better an engine was, the more damage rust causes.