I've converted a few of these older triacs, and once you get into it it's a peice of cake to convert to Mach. The stepper drives could be kept, the psu, the contactor, and if it works the vfd. The vfd (brown and pestell) is notorious for failing for no reason, and can be replaced with a modern vfd.
All you need is a good breakout board and an isolated 0-10v analogue board for the vfd. If you need any pointers give me a shout.