Hi New Zealand.
Unfortunately cannot help you with extraction method from 1912 M3W but fuel tanks leaks are fairly common in amongst four wheeler fraternity as the tank can flexes a lot. Generally they are soldered, unless yours has been remade perhaps in stainless steel.
If fairly original once you extract it take it too a radiator repair shop that does fuel tanks as well. Do you have contacts for the older M3W in New Zealand? There are a number who should be able to assist. If not get back in touch with me and I will look on the membership list to hopefully point you in the right direction.
Regards
Doug Lloyd Tauranga.
