Terco has been out off business for a long time.
You may have to retrofit your mill with new stepper motors and driver-controller.
Doing a retrofit will also enhance the machine tenfold.
Gecko is coming out with a inexpensive drive and finding new steppers wont be a problem.
Mach 3 is also inexpensive and works very well.
Then you have to chose a cad cam program.
There are several Terco owners here and I am sure they will help out.