I don't know of a commercial after market ventrui for the Saito .40. Saito did make a C/L version of that engine, so there should be stock parts available for it. On the Tulsa Glue Dobbers web site there is a section by the late Bob Reeves on how to modify an R/C carb to use it as a venturi, but while it works very well,  I don't think you can put it back into stock configuration again. You can try it like it is, but close the throttle about 1/2 way and lock off the barrel. This will closely mimic the choke area of a restrictor C/L venturi. Otherwise you will get a run that is all over the place.
  Type at you later,
   Dan McEntee