Is this a 2-stroke or 4-stroke engine? 4-stroke Tecumsehs (usually branded Kirby Lauson in this part of the world) are common on very old lawnmowers, and the carburetors used should be fairly easily obtained just by carrying out an act of euthanasia on one of the mowers, which would be a public service. There is a good chance the carburetor will suit an outboard. If the engine is a 2-stroke it is relatively uncommon, and I don't personally know whether they used the same (fairly crummy) carburetor as the 4-stroke. However someone on this board might know - I think I have seen at least one post referring to those engines.
