John, make your own voltage divider and don't fuss with expensive power supplies. You just need a couple of resistors picked up from an electronics store. Consider the following formula for a resistive divider:
Vout = (R2/(R1 + R2)) x Vin
Where R1 and R2 are resistors in series, Vout is output voltage (12v) and Vin is input voltage (24v).
You can see that you'll get 12v output if you tap between two 10 ohm resistors in series like shown in the attached diagram. In your case, Vin is supplied by the wire going to the coil. Splice the resistors into this line with Vout now going to the coil. You'll have to scale the resistors to the wattage of your 24v input current, otherwise they may overheat. I can't recall what it is on the 24v system but you can measure with an ammeter. The rest of your system exclusive of the ignition circuit will remain 24V.
You'll want a protective housing/heat shield for the resistors. I used a soup can cut down to ~the size of a hockey puck, drilled several vent holes and mounted it in the air intake horn so it would be cooled by the fresh air stream. It worked great and I never had a problem. Resistors are so cheap that you can afford to by a bunch of extras to pack with you in case you need them. If a fancy power supply fails out in the bush, well I'm not sure what you'll do then!
Cheers.