As 1120 has been getting closer and closer to staying on the track all the time, I’ve started to think about making her less like the Maryland &Pennsylvania engine she started as. The plan is to 3D-print some eye candy to help convert the post-1900 Baldwin product to a reasonable facsimile of the 1890 Brooks engine.
The domes are the principle spotting difference between manufacturers, and as the model comes with spare domes, I assume the installed ones are removable. These are the first parts up for replacement. There is also a difference with the stack, but there are enough other inaccuracies that this doesn’t bother me too much.
The Bachmann model comes with the headlight where the number plate should be, in the middle of the smokebox door. In 1905, the engine probably didn’t have a generator for electric lights yet, and so, I took the oil headlamp from 2543 and thickened it so the Bachmann LED will fit inside.
While Bachmann provides a choice of pilots, neither is the horizontal-slatted variety favoured by the GTR after about 1900. The pilot steps are integrated into the Bachmann frame, but GTR engines didn’t have steps because you could just scamper up the horizontal slats! I think I should be able to cut off the steps themselves, and use their metal supports to lend some strength to the printed pilot.
Finally, the engine I bought has dual air pumps on the left, while 1120 had a single air pump on the right, which was mounted without any thought to future modellers converting Bachmann engines. On the real engine, the running board had to be built around the air pump. Drilling and shaping a big hole in the running board of a finished engine feels like a recipe for disaster. Fortunately, I don’t need the air pump to work, and so I will simply make it in two halves and try to mount them vertically relative to one another.
I have little hope that the parts will fit on the first printing as they are all based on dodgy measurements of the model. I didn’t even take the engine off the tracks! However, this is the beauty of 3D-printing: experiments are cheap.
Pembroke:87 
I’m not sure what the tricky bit is with mounting the air pumps through the running board, so this suggestion may be a non-starter, but . . . It occurs to me that something like a 1/16″ drill bit would cut through the running board right up against the boiler and would allow you to create a hidden mounting pin and socket in the upper and lower parts of the pump, to ensure alignment.
Thanks, Rob. I did think about this, but I wonder if the risk is worth the reward. I believe the running boards are diecast metal (?). Well, maybe I will just see…