Centrifugal Governor
Registered

Thank you for those links, with lots of helpful suggestions.Nice description of your build.
I think those english instructions were written by me. (I found that scanning the german with the scanner set to 'german' meant the umlauts, etc., all came through in the resulting PDF. I could copy the text to google translate and get a quite a good result very quickly.)
The only thing to add is a flame/heat shield on the front over the spring. There's a slot at the bottom of the boiler front-plate, and the heat exiting that cooks the spring. Described here:
Regner
Photos of the heat shield in this thread:
Lighting a butane fired locomotive
The latter thread also points out that you can file the end of the chassis to allow the body to be lifted over it, using only the 2 bolts on the sides to hold it on.
I have been able to use my phone to a certain extent, to scan & translate the German text. So far, the flame has remained in the fire tube since fitting the air control ring on the burner. The answer seems to be with not over firing the burner so it seems a watchful eye is needed to glance down the chimney every now & again to make sure the flame is burning in the fire tube & not in the smoke box.