You may use almost any master you can fit to the handle-bar….differences to check for, are; internal piston diameter, hose-connection/angle, fluid-chamber angle, brake-light switch connector…. -the first (piston-size) will be the most important.
Old and grumpy,(?), have fairly wide engine & tuning experience, "restoring/resto-mod" on cars & mc, worked as electronic tech. (retired).
GT125R - sold. New (used): GT250R-efi