See if Dave Clements will sell you a OM Hammer/Trigger, Not so sure he will just sell the parts anymore. Give him a call and ask, I'm perty sure he will do the conversion for you.
Or there is the homeboy way.
Here is a my OM 45 Colt Brass Bisley,
Clements Hammer/ Trigger.
No good picture of the Homeboy .22 Single Six,,, but if you look real hard bottm right you can catch the hammer.
You'll need to fabricate a "carrier" for the OM design trigger spring to work in the New Model brass grip frame. ( I'm not certain, but I believe the Brass Bisley grip frames were only produced in the New Model style whereas the "other" brassies have been produced in both New Model and Old Model styles). A series of images taken during fabrication of one of the adaptors appears HERE.