First, use the Internet - where you are now - to search for MKT steam images.
Just cruise around. Take your time. I just did a very quick search and found images of
a MKT Pacific type that, while it wasn't a USRA design, could be emulated using any
of the available USRA Light Pacific models that have come from Lionel, K-line,
Weaver and MTH. K-line even offered a high-quality sub-scale (1:58) Pacific that was well-made and detailed, and geared for good running. Da Bay is the place to look.
Second, you do not have to paint the loco, unless you buy one in an odd color. Don't do that. When (and if) you locate the decals, all you have to do is remove the existing
numbers/letters and apply the Katy items.
Third, fingernail polish can remove some lettering, but not all. Fine "sanding film"
(hobby sandpaper - Hobby Lobby has it) can be used to gently wear the lettering off
without affecting the paint too much, if at all. Overspray can also work. The new decals and the flat (Dullcote) overspray will bring it (the finish) back to consistency.
A black steamer is a black steamer; you do not have to paint it black - again.
There - the basics. Re-decorating a black-painted, brass or die-cast model steamer is one of the easiest modeling things that you can do. And I'm not talking about a so-so job; these methods can yield very nice results. I know.
Cruise the Internet and auction sites for the engine, photos and the decals.