I’d look closely at the motor and see if it is 12v.
If not swap in a 12v motor, search this section for info on 12v motors, easily found at local auto parts store and not at all expensive.

Oops, not sure where to get one in Japan, but you could buy one from the US

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-