Albion - I apologize for the somewhat abusive reply, but I did not understand your intent from the original post. 12V starters ARE apparently available as mentioned in the above post, so you could either go with either a 12 v or 24 v starter. Just remember as mentioned above, all your 12 accessories are going to draw twice the amperage of their original 24V components. If you original wiring system is reasonably intact, it may be more cost effective to get the missing 24V switches and keep your system at 24V.
For example, most all of the electrical connections are routed through the three way headlamp switch, about 12 connections in all. Trying to rewire all that will be a bear to convert, not to mention what do you do about your 24V in tank fuel pump, flame heater, air horn solenoid, gauges, personnel heater motor, gauge lamps, front axle switch, low air pressure switch and buzzer and many other unmentioned other 24 V components?
I replaced my "ignition switch" technically, called the accessory switch, with an aftermarket ignition switch from my local parts store. Cost about $12, has off-on-accy-start positions, so you don't even need a starter switch, although I do have a stater switch and jumpered the two together so I can engage the starter with either the ignition switch or starter switch. All it took was a little work with the Dremel tool to convert the original round hole to a "D" hole ans a few bullet connectors and I was in business.
Replacement 60 amp 24V alternators aren't all that expensive, probably cheaper than a new 120 amp 12V alternator, in fact I have two spares on hand which I can have tested and sell you one if need be. As I stated above, if your wiring harness is reasonably complete, it should be far cheaper and much less labor intensive to buy the missing 24V components and keep your system at 24V than to convert everything over to 12V.
Exactly what 24V components are you missing? Someone here on the list will have all the pieces you are missing, and at prices much less than you need to pay for new parts. All you have to do is post the parts you need, someone will have whatever you need. I am sure you can keep your truck intact with its 24V system at less cost and a lot less effort than trying to convert it to 12V!