DIY VR Treadmill

I got a Dynamix 6EPHU Motorised Treadmill from ebay for £20,
with the intention of modifying for use in vr.
one main function I need is motor control based on my actions
Begin walking the speed to ramp up to match my pace.
stop when I do. Any advice on the best way to implement this.

You could mount an Ultrasonic Ranging Module HC-SR04 on the control panel to measure distance to your torso.
Post #4 in this thread has sample code for a PIC micro controller.
https://www.electro-tech-online.com/threads/16f1827-code-in-asm-hi-tech-c.136326/
I'm sure you could find it for Arduino as well.
Just wire the micro controller to close the speed plus and minus switch circuits.
Do not disable the original control panel or you will open a whole new can of worms.