The 3rd Rail High Iron K4 is a little large for the GN locomotive you are looking to replicate. Also, GN's design of the Belpaire fire box is a bit different than the PRRs. GN's appear to be a little boxier than the PRR versions. It looks like the drivers are somewhere perhaps in the 65" range? Just a guess. As other posters have mentioned, an older AHM body with modifications and detailing mounted to a traditional sized chassis for the motor and main drivers might be the best route. You can always change the pony trucks out. Cool that this little Pacific had a booster engine on it. Definitely worth capturing in one way or another.
Another route that has been suggested might be to search for an older 2 rail brass donor as a starting point.
However you figure it out, that is a nice small Pacific that would be great for smaller scale layouts.
