PJ,
The OEM CB200 mechanical disk looks like the caliper is mounted similar to the SOHCs and others with that floating/waving arm. Is there any way to graft say a CB500 or 750 or CB360 caliper and mount onto the leg? I don't have either to check on, but that might be easier than say an EX/SV swap with an adapter.
Going with a more modern caliper needs a different mount to pick up on the three places on that fork leg to be secure. One 8mm for strength and the two 6mm fender bolts to stop it rotating.
On a drum brake 200, the conversion is probably a complete Cb360 front end