Try Kevin Browne aka KB Cine Repairs in Nottingham, who might be able to source you a module or suggest an alternative.
Failing finding an original, there are generic frequency-to-voltage converters that can be adjusted to suit a range of magnetic tonewheel tachos, to provide feedback to a standard DC motor driver module. This is expensive but just shown as an example:
https://uk.rs-online.com/web/p/emi-f...ories/0440509/
There are doubtless direct-from-China equivalents cheap enough to take the fun out of building your own.
See also these people, IIRC we've had some controllers from them and they seem quite flexible and open-sourcey:
https://www.4qd.co.uk/product-category/controllers/
The induction-motor projectors use stepped pulleys on the motor and main shafts, with a striker operated by a lever above the film track. Hence the warning printed there only to change speed with the projector running. The earlier machines with universal motors switched electrically between two centrifugal governors.
Coincidentally, I also had problems with a projector speed control yesterday - a Lenze inverter that drives the 3-phase motor in a Philips DP75. Proj kept on running but lots of smoke came out. Rifa suppression caps, naturally. Grr.