Hi Keith,
The answer to your question depends on the condition of the soundboard, but I can tell you it is rare that a soundboard of that age has not deteriorated. Most Mason & Hamlins from that period spent their lives between Boston and Washington DC, and this is one of the harshest environments for pianos in the world. If your piano spent its life in CA, or NM, or AZ, there is more hope for the soundboard.
More likely than not it still has ample crown. Mason & Hamlin crowned the ribs and the inner rim during that period and they usually retain there crown, but that does not mean the soundboard will still work properly.
If you want to restore the piano to its original condition, that means new soundboard, pinblock, action and finish. FWIW, I owned a 1917 AA while I was a music student, and that piano was really a wonderful instrument.
We recently restored a Bechstein from the 1890s and saved the board because it was so pristine, and it turned out great. That is the exception IMO, and not the rule. We also restored a similar Bechstein that was about 20 years newer, and replaced the board, and as good as the Bechstein with the original board turned out, the one with the new board ended up with more power, range and sustain.
More than 60% of the rebuild work we do is on pianos that have already been significantly worked on within the last 15 years, where the original board was not replaced. These pianos come to us because the owners are dissatisfied with the performance of these pianos, and are ready to do them right. This is quite expensive. The rebuilders who can do your piano justice will charge 25k - 35k. Your expectation should be that your totally rebuilt M&H should equal, or outperform its new equivalent, which is a tall order since the new AA is really wonderful.
Another reason to replace the soundboard in your piano is that the original belly design on the AA has a wonderful bass and midrange, but a weak treble. If properly rebuilt, this can be corrected, but only with a new board, and the right rebuilder.
Please feel free to contact me if you have more questions. I hope you do rebuild your wonderful piano at a level that it deserves.