I don’t understand why that rockerbox would be so difficult to remove, is it just the resealing of the gasket area that concerns you?
Anyway, if you’re determined to keep the box in place, then I can see only 2 realistic approaches:
1 – chiselling, but rather than using a sharpish chisel from the beginning, you need to prepare a surface for a blunt drift to hit. This could be done with a file or careful work with a small sharp chisel. This will give the drift better impact transference with short, sharp Shocks
If this doesn’t work, then
2 – cutting/grinding the cap into pieces so that you can collapse the remaining threaded collar enough to release its grip. I cannot imagine doing this without much horrible material getting into the engine (including grinding wheel abrasive particles, if you employ such a tool). Even hacksaw blades and files leave nasty bits, that’s why they wear out.
If I were to do this (which I wouldn’t) then I would want to strip the entire engine, not just the rockerbox.
If the chiselling method in #1 above doesn’t do the job, I would just bite the unpleasant bullet and remove the box, get the job done and worry about resealing the box afterwards. You may come to regret taking a shortcut because of lack of time.
Unless you are going to get another unusual cap made, it is likely that you will need some new standard caps.
I believe they are the same for all pre-unit models (other than 3T) from the very beginning to 1962. The parts manual
for 47/46 shows them as E1564, and they look the same as the ones in my 1960 engine, which have the same part number.
No where are early iron caps mentioned, I am doubtful such things ever existed.
Those caps are readily available in the UK, I would imagine that CBS and many others would have them in the Americas. Of course shipping to Uruguay may have its difficulties/delays.
Apparently the caps are also used on some other models and marques, as mentioned in these links:https://www.aceclassics.co.uk/products/pre-unit-triumph-rocker-box-caps-e1564-1938-1962/https://www.feked.com/triumph-pre-u...-sump-plug-on-unit-350-500cc-models.html
Best of luck!