[Arran]

For the Japanese radio, a wire wound resistor screwed to the chassis will be the simplest solution to get 100v to the primary.
Dave

Dave;
A resistor may be a simple solution but I wouldn't call it the best, heat is the enemy of electronics and if you can avoid adding more of it it's a good idea. Suitable bucking transformers are just about everywhere for the taking, and all of the other parts needed are available from the local hardware store. I'm the sort who prefers to find a way to operate a radio as it was originally designed, even if using modern parts.
Best Regards
Arran