Update:

I see you have 5.32 vdc with the new battery with the headlamp on. That is a near normal voltage. It seems you have eliminated the issue that had the battery at less than a volt with the headlamp on. When I swapped engines last summer I had the same problem (bike stalled when headlamp switched on). It turned out to be the points gap and timing. I know it sounds unrelated but that's what fixed it.

1969 CT90 K1