To the OP - there should be some kind of label or badge with a model
number indication "somewhere" on the boiler unit, possibly inside the
outer casing. Maybe an old nozzle with rating ( us GPH and spray
pattern/angle stamped on one of the flats.
Also what flue exit temperature do you have at the point of exit from
the boiler? (somewhere between 200 and 500 degrees C)

Looking again at the plate with the serial number etc. on it, the model
number is Jet 70/90. Now I've looked on the internet at other makes, I
think I can assume that the 70/90 refers to the number of 1000s of
BTU/h. So I think it should be possible to get 90,000 BTU/h or 26KW out
of it. Is this a reasonable assumption? If this is the case then it's
not too far off the lower end of my estimate - I have been a bit
generous in places!