OK, boys, time for the dreaded "Reality Check" (not to be confused with another type of check, which we will refer to shortly).
First, there are NO pieces, bits, parts of steam locomotives hiding in factories, mills, etc. Every one of the 1500+ steam locomotives in New England in 1945 has been accounted for in the last 60 years. There are NO secrets out there.
Second, there are NO plans, documents, specs, etc for NH I-5s or anything else (except for B&M 3713, the 4-6-2 being slowly restored at Steamtown-- as a Lima Loco, the plans exist, because that stuff was donated to California State RR Museum). It ALL went into landfills before most of use were born. NO secret stash of NH steam blueprints anywhere, boys.
Last, you can have any steam locomotive your heart desires. All it takes is many cubic yards of hundred dollar bills. A replica NH i-5 4-6-4 could probably be built for between 4 and 5 million dollars. Where will it run, on Amtrak's NH-Boston route?? On Metro-North?? It's too big a locomotive for practical operation anywhere it would be welcomed, these days. The "new" steam loco will have many differences with the old one, differences in fabrication and machining techniques, safety appliances, etc. So, the "original" plans would be a good guide, but not an absolute roadmap.
The "Tornado" just completed in Great Britain was a wonderful project, and one that will never happen in the US, because USA railfans are too cheap, too provincial and too lazy to make it happen.
If you want to see another steam locomotive operating in the region, start raising funds for RMNE's #1246, a nice light Pacific built in 1946 (relatively modern). There is a shop building and a 20-mile railroad to run it on. When you have collected about $700,000, call them and they will be ready for you. Then, 2 years later, you should be able to blow the whistle. And if you raise the money (or most of it), rest assured you WILL get to blow the whistle any time you wish.
"You can't always get what you want, but if you try sometimes, you just might find you get what you need."