If I were handed the gun and said "do something with this", I would restock the existing parts with a new barrel and piece of wood. In truth, a lot of octagonal barrels have hogged out rounded barrel channels, but fitting an octagonal barrel to a stock that is already shaped may prove more difficult than its worth. It's probably doable, but I could see it being quite a headache too. Better to start fresh with a new piece of wood and a proper barrel.
-Eric