I'm in a similar situation; trying to convert an H-4 to a BR&P LL-1/B&O KK-4.
Been working on it, but moving pretty slow.
I removed the details, took off the smokebox face and filled in the holes.
Unfortunately, my eyesight etc. isn't what it used to be.
I "lost" a bunch of those tiny rivets around the outside edge in the process of cutting/sanding off the current door. Bummer; and not sure about fixing that little goof...
That's about as far as I've gotten, but I've been thinking about the new door too.
Assuming you don't want to make the door from scratch, and depending on what you are building (and how proto it has to be)-
Have you considered modifying a boiler-face from a small switcher, or even a narrow gauge loco to create the door?
You might be able to mold a copy, or get a part from Republic Loco Works that would do it.
If it's small, the whole face might work; otherwise you might cut/sand off the excess & just save the door from the other loco face.
(I've done this a couple times & it worked pretty well)
As for number plates, I have a set of etched brass steam loco parts from Gold Medal Models.
It has 2-each of 14 different number plates, including some road-specific ones.
And a lot of other useful stuff if they are still around and making it...
