I think the easiest way to go is to try to buy one of the OS venturis or any other engine manufacturers that have venturis available. The Tower hobbies website is good about having the dimensions of these items. Just measure your throat size and find one that matches.
Or, if you want to make your own, I have used brass tubing with smaller diameters inside to create the size I wanted. Of course the best is to use a lathe to turn them perfectly to size.

As for the needle valve, you should be able to find Super tiger needles available on line somewhere. They seem to be the standard and work very well.

I am interested in the performance of this Magnum engine. What type of flying will you be doing with it?