Geology 553 boom extension problem

We have an old Gehl 553 that has a broken chain in the boom. There are 3 chains , 2 for extension and one for retraction. We think the last user was cutting trees and dropped a tree on the top of the boom and broke one of the “double” chains. We can see both ends, on top front of the boom and inside of the back inside of the boom. Does anyone know how to replace the chains. Do we pull the boom out or is there a way to fish a new chain in.