I am just guessing here, but I would expect a venturi the size of the OS .20FP should work fine. (I don't know the measurement, though)
I'm upstairs, but if someone else hasn't answered by the time I get down to the shop I'll send some dimensions.
Search on venturi size, you should find lots of posts.
Remember that the important thing is the venturi
area; for these dinky little engines with spraybar-style venturis the area is heavily affected by the spraybar diameter -- so if you just match someone else's throat diameter without taking their spraybar size into account, you'll get it wrong.
(and to complicate things, if you're building your own and you get the spraybar significantly off-center, you'll end up with a bigger venturi area than you would have with the spraybar centered).