TBH for a field bike you may as well stay as you are, you would need the flywheel, stator plate, coils, pick up, sub loom and cdi unit. All this needs to come from a 78 on dt 250/400. I don't know if the flywheels are interchangable either. They are on the 175 if you swap out points for cdi but i don't know on the 250.
Neither of these bikes (DT250 or DT400) are all that common and price wise i think you would be looking at £50 plus for the generator assy, stick a chunk more on for the cdi, plus wiring and you have a bike no faster than the one you have that you can't fix in the field, so to speak.