The 4 wire rectifier from RS is a single Phase ac to dc rectifier/regulator. You have to float a ground on the stator and run your floated ground and stock a/c output wire into the reg/rec (2 yellows). Red wire coming out is the 12v charge wire, and black coming out of the reg/rec is the new system ground.
Inside of the flywheel spins around the stator and induces an ac voltage, when the outside pick up coil senses the magnet going by, it produces a small spark as a sort of crank sensor. Thus triggering your to cdi to tell the coil to fire the a/c voltage off and create a spark at the plug.
Other common problems are tors wires shorted or connected together, or the tors system not being properly removed. I personally have been messing with Banshee's since 1993 and have seen I think maybe 1 cdi go bad out of the fleet of bikes we own and that I work on for people. Without a strong sense of how the electrical system in the bike works, it can be very hard to trouble a no spark condition. Even if the bike had the malfunctioning component replaced, if you removed other parts improperly to get to it, or installed them improperly (ie flywheel or pick up gap) then you will still have no spark when your finished.
You need to make sure the tors is propely deactivated, your stator lighting circuit wires are properly installed, your flywheel is not cracked internally, your pickup gap is correct, your kill and key switches work right, you have no shorted or open wiring, and all the connections in the system are not corroded. Using an ohm meter to verify circuits and switch operations is vital and rather easy to do. If you pm me I can help out further, but this thread somehow turned into a lighting thread.