Herb,
I really doubt it will be too difficult to get approvals to operate #12.
Missing documentation is a way of life with these things, after all, We didn't have any documentation on the 19 or 20,Patches, replaced sheets, you name it, we've dealt with it all here it's just not that hard to create, especially if you'd built the boiler in the first place.
I discussed the #12 with our regional FRA inspector years ago, Basically, we would have done a boiler survey, material testing and the stress calculations to create a new form 4, same as our other locomotives.
Time will tell........ Jerry