"The advantage for the E12 is that the motor doesn't have field weakening, so it's about as simple as you can get." Having done a few curtis conversions on e15/e20/I-5 i can say that it is very easy if you want to completely rewire the machine. I had plans that i sent out into public domain almost a decade ago. All you need to make sure of is that you have a controller with high pedal disable, and put a 3a fuse on the field.
I thought about this last night, and true: The E12 motor (at least the one with a field) *can* be run with field weakening, but it's not supported by the simple contactor controller.
The E12 with the permanent magnet motor of course can't do field weakening unless you remove magnets (so not advised :-)
C