This may sound wrong, but..

After a bit more thinking, it has recently occured to me that using a modified Hank to make the Hogwarts engine would actually give a more accurate result than Henry ()

Why? The firebox is larger, and the boiler is long enough and has that slight slant right behind the smokebox. However, the tender would have to be changed since it isn't correct, and it would be better to use the Fisher Price version since it's loco drive and/or easier to find. It doesn't even seem like it would take too much to convert him for this purpose, despite him having no footplate nor bufferbeam and the physical parts that would have to be removed.






The fake siderods, however, would be cut off and replaced with the normal, working type. Obviously, it would look a lot better in real life, but since I never had a Hank, I have to edit an image of one (though it isn't a perfect illustration). The idea was partially inspired by this homemade Christmas train version, since it also used an American locomotive:

http://www.epbot.com/2014/12/our-custom-...plete.html

This, of course, is the actual Olton Hall:



On second thought, however, Henry wouldn't look too far from that with some cosmetic alterations. Despite Hank's advantages of having the right firebox size and boiler shape and length, the boiler itself is not "slender" enough, as opposed to Henry's where the size is more similar.