Mike E.,
Here is a quote from Matt Gonitzke dated 6/03/08 stating "You'll need either a wiring harness from an 1810/1811, or you'll need to add a relay and a few wires to make the ignition switch work. The major difference between the KT17 and M18 is that the KT has points and a coil, and the M18 has electronic ignition. The reason the wiring needs to be altered is, power is applied to the coil on the KT, but on the Magnum, the ignition switch has to ground the magneto to shut off the engine. No power is applied to the magneto on the Magnum engine".
Just substitute M20 for M18, same engine. I also have a wiring diagram that I got from Steve Blunier. If I can dig it out, I'll send it to you. I used it and my 682 runs great. The part # for the relay is 725-1375.