All dimensions (excluding ball bearings, Bosch magnetos, spark plug threads, etc.) are imperial. Hail Britannia!
Douglas did however use an array of thread standards. You will find BSW, BSF, BSC/CEI, BA, and a lot of Douglas' own proprietary threads. Not only can these proprietary threads have non-standard pitch, they also used some non-standard diameters, like 17/64-25.
The handlebar/head stem bracket was used on other models, so not impossible to find. just beware on some models the handlebars were ahead of the stem and some behind. I have also made them from scratch, as they are not that complicated.
-Doug
[fix typos. 01Jul18. Doug]