It just so happens that someplace a few dead laptops ago I have the manuals for those galvo amps.

The number 651xx comes to mind for the series, and they are NOT online for free, Cambridge sales doesn't do that , you have to call or email, and that design is very old.
You'll need to disable the on-board slew rate limiter with a jumper or turn it down to use those with standard laser show software.
Reason being that system is designed for a constant vector slew rate, so the PID loops on the amps are tuned for that, rather then tuning to the International Laser Display Association galvo tuning. Constant Slew gives you very even image intensity, but does not allow for great speed.
On mine I got to ILDA 10K at 8' scan angle, measured with the standard test pattern without changing the speed or the limiter, but the limiter does, um, unique things to distort beam show and graphic images..... It needs to be reduced in effect. On the other hand it is there to prevent overdriving the galvos in the frequency domain.
So, You'll need to pull the jumper(s) on the amp that connects to the on board D/A converter, turn the limiter down, supply a proper differential input or ground the inverting side of the diff input, clean up the tuning a bit with a scope and you will have, by today's standards, a slow but very useful laser show projector.
Give me a while to dig up the manuals... If not, GSI was bought by Cambridge Technologies which was bought by Novantia, so they may still have the PDFs, but I doubt it. I have two pairs of those galvos and amps.
Those are scanners for precise, incredibly accurate movement, their huge mirror and shaft inertias are NOT designed for speed. If you detune the PID loops too much, the loop can oscillate and either the amplifier or the galvo could destroy itself.
I have both Pangolin and LSX control systems. WARNING, Laser shows are an expensive and addictive hobby, makes the test equipment anonymous thread here look like a lunch money enterprise. You have been warned..
Mine were at the local surplus place. BTW, if your ever want to do precision engraving, or stereolithography, you have the right galvo pair. If you just want to do laser shows, get a pair of DT30 off Ebay for 200$.
PS, Marcan's stuff is a PITA to use, especially if you want to sync with music. Use wav files and a sound card or sine wave oscillators using a sound card. . Yes, there are sound cards that can run down to DC, but the device driver setup does not like it.
You only have enough velocity for about 1/3rd of the stuff in that video, and the wider the angle, the slower the galvos can jump due to device physics.
Given your scanners are slow, you might want to take a look at Maxwell software. Grab a Helios DAC and the free version of LaserShowGen.
If you want to hack, you have on-board, parallel load, 12 bit DACs on the amps. You have an addressable eight bit parallel interface on the upstream control boards , if yours is identical to mine. Ie some address bits for selecting x high byte, x low byte, Y high byte, y, low Byte, and on/off. These are parallel input, and a latch signal.
It also has hardware that blanks the laser till the galvo motion settles using position and velocity feedback signals from the amp.
https://bluefang.itch.io/maxwell Abstract images using sine and triangle waves work better on a constant V system. Ie Lissajous patterns.
Then come over to Photonlexicon where we can help you set up a single mode diode RGB module at around a a half watt of total power for full color.
Aluminum Migration used to be a problem in the laser diode industry. If those are the red model, soldering in a new 35$ diode is an option and the rest of the hardware is fine. If it were a green DPSS laser, the pump diode does wear out over time, and opening up a green and replacing the diode is something most OEMs would dread. Dad might like his laser back.
Keep what you have, OEM cost on those galvo scanners was well north of 3000$ a pair in the 90's. They have ultra precise capacitive feedback on the rotors. As opposed to optical feedback which is probably 30% less accurate in that generation of device.
I may still even have the diode board if Dad wants his laser projector back. But 35$ gets you a diode rated for 150 mW of single mode red.
Steve