Fred; I would try the following :
1) Use adhesive backed paper in your printer ,available in the paper section at computer stores , print out the text in a thin out line and cut'em out by hand. Print out some practise sheets on plain paper .This presumes that your knife skills are up to it , you can get the font or similar to modify .
2) If the font is not available ,it is a common one and something suitable may be printed and then modified in the cutting. You can see in the pic that it is a hand made mask (my belief, not proven) on the real plane as the 'N' just doesnt look 'right' ,beware of the computor 'perfection' it wont look right if its perfect. Isolate,and inspect each letters width o/a and thickness of stroke,and spacing relative to the letters on each side. Stroke is the thickness of the lines that make up a letter ie: the first vertical line in the 'N' these should be similar in all the letters. Do this in the computor if you can, and print off the complete text ,rather than one letter at a time , if you can't, draw on the print out to suit .
3) Layout the whole sheet full of text then you can have some practise masks ,leave a border of at least 1/2" around your copy to mask up to. I would put in a 'window' if I sent it to my plotter.
4)Layout on the model is gonna be tough as well ,place the main body then add in the centers of the 'A' . Burnish diligently the edge ,spray immediatly ,and dont load it with paint ,spray ' into' the letter instead of out of it. Practise to see how the paper reacts to the paint ,LOW pressure and the LIGHTEST coats possible. I have been using a timer to keep myself away from coating to much too fast ,its hard to stay away! I read recently that sealing the mask edge with clear will stop the paint creeping under. Allow coats to dry!! This will be the sticking point for this procedure so be diligent!
5) You can also use a copier to up/down scale the pic and trace the text ,transfer that to the adhesive sheets and go to cutting.The font has no round edges so its an easy one.
I think your skills are up to this Fred ,I saw your Stuka and your X-15 .This is a very good skill tester and is excellent in the fact that they may be the only decals of this type on the planet! I think its best when something is NOT readily available ,it will be unique . A good addition to the build sheet as well.Tricky but very doable and a feather in the cap for you. There is nothing new in modeling ,there are well-proven methods and combinations of procedures that have been used for decades,this is one of those.
Some font trivia now : When fonts were 'digitized' computor software designers cheated the font designers out of licence fees by changing a certain percentage of the letters in each font they stole, the trick is finding out which letters have been modified.
If you still want me to custom cut for you I would be happy to do so. Please send me the font location.
Step 4 is the very tricky part ,if you can do this then you will have the painted markings you want. Think about the faded paint as well ,although BA planes were pretty clean .