Easiest fix is to cut the balsa from bottom side half way through same size as new pinned hinge. You can then place new pinned hinge there epoxy in. Put piece of balsa over it glue in. just recover or paint and all set. Lot less work than cutting off all hinges and replacing. Another simple means would be to use a monocote hinge alongside old broken one. Since you still have 3 good hinges I would not be that concerned with a major replacement.