It's a Doppler pulley. I think you can use the stock pedal crank, but you may have to get new circlip grooves turned into the shaft to get the pulley in the right spot.
You don't even have to balance the weights that much. I put so many crazy combos in there, it doesn't even begin to matter. You can put them all on one side if you want.
The total weight in the variator determines the RPM at which the cheeks close together. Force=mass x velocity or K=MxV. The force constant(k) in this case is the tension of the engine spring and the velocity is the engine RPM. I'm not saying you have to do math, just getting some perspective in there.
I run 15g against my doppler spring with the 70cc H2O kit, and like 12g with the 50cc kit(it revs higher). With a stock spring you'll need less mass.
Oh, brilliant and obvious engine spacing method i came up with while biking to work this morning: 1" chunk of aluminum(or whatever, wood maybe?) with two holes drilled in it and some longer socket cap screws, and just insert it between the doppler or stock spring and the frame mount. Done. Just get a longer belt and figure out how far the engine has to move to tension it, then cut that thickness spacer.