Hi, Monty
Making a new disk is the way to go as suggested above.
On my Emco Comp. 5 lathe, I used the existing OPTO sensor/pickup, Only needed to add a current limiting resistor to power the OPTO sensor properly.
Original Disk was about 5 inch in dia., With the single slot width being about a 1/4 inch, Increased it to 7/8 inch and It's good to 4000 rpm.
Mach3 just need's additional time/width to see the change of State of the OPTO sensor at your max spindle speed.
Nice looking Lathe, You haft'a do what you haft'a do to u-load these machine's.
Hope this Helps, Chip