There was available a kit of NSB class 21 for some (25?) years ago, but I doubt if it is available any more. To get hold of it, assembled or unassemled, you have to try various Internet forums (like this one) to see if anyone are willing to sell.

A comment on the "design by a Sctosman". I wrote an article for some decades ago, probably with the translated title "David Jones Locomotives" which where about classes 11, 12 and 13, which we at that time (1980-ies) thought were constructed by greate influence of David Jones at Highland Railway i Scotland. We saw some similarites between the Higland Railway Jones Goods, a 4-6-0 engine built i 1894 by Sharp Stuart. The first Norwegian engines of class 11 (which is a forerunner of class 15, which again is a model for class 21) where built by Dübs, Glasgow. These enigens where however built before the Jones Goods engines, and the specifications given by NSB chief engineer Oxaal to Dübs, are very detailed and with little room for modifications, so the connection to Mr. David Jones is probably exaggerated, if existing at all.