Repro iron clutch masters are available at the vendors. Problem is, DeTomaso's mounting bolt (or stud) spacing was non-standard so non-factory cylinders don't directly bolt on, there were never real rebuild kits for the stock units and the cast iron used by the factory (and some repros) seems particularly corrosion-prone.
A very pricy fix is to have a shop like White Post sleeve a stocker with stainless steel. And if you know what you're doing, you could have them resize the bore to take a commercial piston and seal, thus making the sleeved unit rebuildable. That would take considerable research.