I asked a very experienced rebuilder about making a push up rod for the Carter updraft BB-1. They are not available anymore and the thought was that perhaps we could make it ourselves. Here is his answer:

Sorry, no, I don't have the spec on that rod. The diameter is either 3/32" or 1/8" IIRC, and it should be tapered somewhat on the end which goes into the power valve in the bowl bottom.

To determine the right length, take a piece of brass rod and push it down into the power valve to compress the check ball spring. With the spring compressed, mark the rod at the exact point where it is level with the top of the bowl casting gasket surface. Now invert the bowl lid and place the rod into the vacuum piston hole but DO NOT COMPRESS THE PISTON. Mark the exact length from the piston to the bowl lid gasket surface. Measure the two marks on the rod and add the two figures plus 1/32" for the gasket thickness and you should have the length to which to cut the rod.