the basic treadmill is a collection of parts.
the motor is powered by a driver. the driver takes mains voltages and by means of some signal, regulates speed.
the older/cheaper ones use a simple voltage signal, often a 10k pot or suitable PWM alternative to set/change the rpm.
the user interface board is more like a tablet. just some fancy displays with lights and sliders and such. nothing to really to with the motor. you can program a journey to match some video. go up a hill and the motor slows and the incline alters.
go down a hill and the reverse. so the user interface is isolated and not really needed for motor operation.
all that fancy display stuff and there is one output to the motor and one output to the incline.
there is also a power supply. like your PC, it is off by itself and provides power to the other boards.
it may have a relay for up/down on the tilt motor (if there is one)
back to the motor driver. it probably connects to mains voltages and runs the motor on 48 to 90 volts
taking the 3 main boards, user interface, motor driver and power supply.
what one is the problem ?
if your power supply took a spike, it may have wiped out the other two. but the driver is very hard to kill.
if you can isolate the motor driver, probably hidden under the chassis. and figure the control input, you might be able to replace it with a pot.
if you can, get a simple servo, connect it to the pot and keep your stuff isolated !!!
rotate the pot by using the servo.
one leg of the pot might be mains voltage. yup, the driver is crude, electronically speaking.
so, follow the power cord, the first board it comes to has a transformer and some relays. = power supply
trace the wires from the motor = driver
user interface is on top.