Looked up Mall Drill;
"In 1956, Mall Tool Co. was acquired by Remington Arms Corp. and renamed the Remington Power Tool Division. In 1969, some entrepreneurs purchased this Division and named the it DESA Industries. They were acquired by AMCA (Master Consolidated) in 1975. Parts and service for Mall Tool products are no longer available."
Maybe similar trigger would fit mount & opening, if you got another drill for spare parts...
But figure why the trigger is missing, maybe drill motor is burnt out & it was used for spare parts...
Good luck!!