I picked up a mostly complete plastic static kit of the Southern Pacific RR's first locomotive, a 4-2-4T known as the C.P. Huntington (preserved today). This is a tiny engine even in O scale, it's about the same height as the Bachmann On30 Mogul, maybe a little wider, but shorter than the Forney.
Having trouble figuring out who made the kit, I think it was MPC that did this and a General 4-4-0 in the 70s but no one's had one on eBay lately.
After comparing it to an early 60's Aristo-Craft HO Reading 4-2-2, I think I could turn it into a 30-inch gauge inside frame 4-2-4T without a lot of trouble, just by cutting up the frame from another of those 4-2-2's (I have two more that have lost tenders in various stages of disrepair) and doing a little trimming and filing. The main frame under it is just slightly wider than an HO wheelset. The 4-2-2 drivers are tall enough to do justice, the only tough point is making counterweights and so on that all work. But it would look reasonable. I suppose some extra pieces from a Bachmann Forney (trailing truck) would make it work.
Question is, was there any similar prototype for such a thing? I'm sure it would have been fairly limited in pulling capacity and slippery with just two big drivers as it was. But if it will pull two cars that's enough.