I suppose the best way to think of this project is a kit of parts. With new Holland Double rifles now being six figures and good used ones at auction are making Ł20,000 plus, there is potentially plenty of value to extract here. With a new forend and lock made by a third party it will never be a “pure” Holland, but how many of Holland’s actual gunmakers were / are in house compared to out workers who are used by many in the trade.

The one real big challenge would be engraving on the new locks. I expect replicating a full hand cut Rose and Scroll would be a challenge, particularly as every individual engraver had his own unique finger print. But given lack of engraving this challenge is somewhat reduced. And what is the state of the barrel’s- assuming still good.
And only the OP knows whether he has the skills to make the locks - hand forging, then filing up and heat treating all the required parts.

Whatever the case what a fascinating project. Whether you make money on it really is your call and depends on how you price the time. Whether it is worth spending Ł5000, Ł10,000 or 20,000 on a gunsmiths time is the call of that third party. Its a new Blaser, Heym or Merkel double rifle or a rebuilt Holland.