I am sorry I took my eye of the website after my original post; I hadn’t expected such a welter of responses.
I had refrained from mentioning specific companies but, yes, my VM30 was supplied by Allen’s. Overall the service was OK but the man who answered the phone always seemed pretty grumpy, as if he didn’t want to be disturbed. Very knowledgeable though.
Most people seem to have found that light dressing of the OD of the Mikuni inlet stub will enable it to fit the MZ inlet manifold. I don’t think that the difference is as much as 0.010″. This would be best done in a lathe but I don’t have a 4-jaw chuck so did it gradually with 240 grade wet and dry, with frequent trial-fits. It’s almost impossible to get the stub both round and parallel by hand but I obviously did it well enough though, I confess, I used a smear of silicon just to make sure that there would be no air leaks. So little metal needed to be removed that I am sure the carburettor could still be used with the usual moulded-rubber connector.
The standard connector to the airbox is of a decent wall thickness (5-6mm). I followed the advice I had read elsewhere and gradually whittled sufficient rubber from the ID using a craft knife. It sounds crude but with care it can be enlarged fairly accurately. A decent worm-type hose clip clamps the hose snugly around the carb inlet. A stepped adaptor would have been more elegant but I didn’t have the necessary diameter aluminium bar or tube to hand and, besides, I think that it might have taken several goes (and a lot of swarf) before I got it right.
Incidentally, as everyone else probably knows this, backing off the nuts on the top engine mount allows the engine to pivot forwards and downwards (don’t forget to slacken the exhaust hanger first). This makes it much easier to fit the carb and, if necessary, to remove the throttle slide and needle with the carb in situ (although I found that the two manifold screws still needed to be removed to allow the carb to twist).