You will have to setup a complete seperate circuit for the 24V system including fuse boards.. etc.. you will also need to make sure everything is rated at 24V... It'll be a compicated setup but not impossible... the early series landies had both 12 and 24V, so maybe if theres one around where you are you could take a look.
I had no luck looking for diagrams last night
Best bet is to speak to your local auto electrical shop.... they should have/be able to order everything you need and suggest a way of connecting it.
The other option is a transformer to change the voltage from 12 to 24 but that will be inefficient as most transformers loose at least 10% of your current....
Everything I've read online however, suggests you swap the motor for a 12V one.... as it'll be easy. Although they do say the speed of the 24V motors is far superior.
My car currently has numerous voltages as I have lots of strange stuff in it ( I use transformers tho) oh and it could really do with another battery ..
1. CB Radio - 18V
2. 5.8Ghz Radio Receiver - 24V
3. Laptop - 14V
I bought some of those cheap transformers.... but for a 24V winch the transformer would cost alot... but if you know the Current it draws DSE would probably have one.