I found that searching on ebay I was able to find alternative main jets for my VM28 Mikuni (no idea what it came from). There were two types commonly available but taking the old one out I was able to clearly identify the type I needed. I have the carb fitted to a Supa5 engine in my ES250 and it works well but perhaps a tad thirstier than the original BVF with the settings it came with. I have just dropped the main jet from 140 to 135.
Incidentally, I have found two things which may be of general interest. Firstly if you saw off the clamp on the MZ manifold and file the stub round, you can use a piece of radiator hose to link carb to manifold.have doen this for my ES250 and for a supa5 manifold. Makes switching carbs a lot easier and no air leaks. Secondly, the ETZ125 hose from carb to air box has a smaller diameter suitable for the Mikuni and is a straight swap.These two mods avoid the need to make up sleeves.
Hope that helps.