this truck, when equipped with the correct mixer, uses a diaphragm that has a different PN than your generic ca50 mixer diaphragm, and actually has a thicker washer under the "bullet" which keeps the "bullet" from sticking in the mixer housing. since it seems your mixer is one with an adjusted screw, rather than a plug and Allen head screw behind the plug, as the correct one would have, most bets are off.
AS you are not in the USA, and as such not covered under the rules of the US Federal EPA, have you considered converting this truck to one with a throttle cable linkage?