The PRR M-1 boiler is the I-1 boiler with a substantially lengthened combustion chamber. I *suppose* you could graft two AF K-5 boilers together and develop the Belpaire contour with a ton of work and/or massive applications of Bond-O. If there is a Berkshire in S, it could serve as the chassis for an M-1.
Alternatively, draw up a CAD version of the boiler and have it printed in a metal such as bronze.