I always look for the easy way out even if it costs a little more. What Loco. are you working on ? Does it have to be a particular number or just different from another one you have. It might be worth searching the parts sites as a lot of Pennsy steamers saw multiple runs. You may be able to pickup a boiler front with a different number.
I agree with Pete on the dry transfers. Usually you can secure the transfer with tape to keep it placed as you start to rub. Say the side of a boxcar. Your talking a vary small area doing a Keystone. Not saying it can’t be done. Decals would be easier. Some sets such as K4 Decals offer the Keystone itself with random numbers to place inside. Probably easier than dry transfers. The only issue. If the Keystone in place on your engine may not be exactly the same as the decal.
I model the Rutland. Their passenger engines had rectangular plate with raised letters at the bottom. Rutland RR. The numbers were above this. Luckily the majority of their engines were 2 digits. They come with the sheet from Highball Graphics and you place the numbers you want in the box. To big to fit on what’s there from the factory. I would paint a sheet of thin styrene gloss black on both sides. Place the number board on the sheet. Let it set and hit it with clear and then place the numbers inside. Doing it this way. You have a large sheet of plastic to slide the numbers along and place them. Rather than dunk them in water. I pin them with an X Acto blade and dribble water from an eye dropper and just let them sit in the puddle till they release. Then position them. Hit them with clear again. I use a pair of decal scissors from Micro Mark and carefully cut it out and place it on the factory board with glue. You don’t have to worry about trimming decals to fit. The final cut out will take care of it.