FYI. Usually all treadle machines use a belt that is approximately 68" (173cm) long & 3/16" (5mm) in diameter.

Fits all treadles Singer and non-Singer. 804
Comes 3/16" diameter X 72" long with clip. Designed to be cut to fit. (Treadle machines all require less than 68")

Quality Leather Belt for Singer Treadle Sewing Machine
Cowhide Belting
** It fits for most singer treadle sewing machines.
Measure your old belt and cut the length you need.
Then insert the end with the metal hook.
Installation:
Measure against the old belt then cut to the proper length. Puncture the end of the belt without the metal hook with a small nail approximately 1/4" from the end.
Push the metal hook through the hole that you had made on the other side and bend over the hook after the fit is satisfactory.

Features: Material: Cowhide leather
Length : approx. 180cm
Diameter: approx. 6mm

Tips:
Tools needed;
a sharp knife or large scissors,
an ink pen or Sharpie marker,
A pair of pliers.
Clip the old belt in two (if it's still present)

The machine should be in the up (working) position.
Fold the new belt in the middle, and place it over the top of the handwheel of the machine
You may have to work the belt around the bobbin winder or belt guard to get it into the right place.
Most Singer machines only had one hinged loop in front.
Be sure the belt gets through the loop(s)
Then work the back half of the belt around the back and bottom of the large pulley

You can work out the size at this time - mark the end that does not have the metal - then you can cut the belt.
Use Pliers to join both ends
The goal is to have the belt tight enough so that it doesn't slip much, but not so tight that it makes the machine difficult to pedal

Leather stretches - after some use you may have to snip some more of the belt and tighten back again with your pliers

To replace the belt:
Loosen the large screw that holds the motor in place (loosen – do not remove).
WHEN YOU LOOSEN THIS SCREW - PROTECT THE PAINT ON THE ARM OF THE MACHINE FROM THE SCREW DRIVER.
To locate this screw; look at the motor as if you are sewing – this screw is below and behind the hand wheel.
Loosening it will allow the motor to slide up and down.
Loosen the small screw on the silver stop motion knob on the large hand wheel.
Unscrew the stop motion knob and remove from the machine.
NOTE the stop motion washer - its 'ears' should be pointing out.
Remove the washer.
Pull the hand wheel off of the machine – the belt will come with it.
Remove belt from motor area.
Clean the backside of the stop motion knob, the stop motion washer, the inside of the large hand wheel, backside of the hand wheel, and the shaft the hand wheel goes on (inside and out).
Put a couple of drops of sewing machine oil on the shaft the hand wheel rides on (inside and out).
Install new belt on pulley on motor. (This is usually a two-handed job.)
Make certain it is on the pulley, riding in the groove, not on the outer rim.
Loop over hand wheel (the hand wheel is not on the machine, yet).
Replace hand wheel on machine. Replace washer ('ears' out) and then the knob. Tighten screw on knob (not too tight!).
Check to make certain the stop motion knob will release (partially unscrew).
If it will not, remove the knob and rotate the washer 180 degrees.
Replace the knob and check again.
Your new belt should be just tight enough to pull machine without slipping -- NOT ‘plucking’ tight! Actually, it needs to be pretty loose.
You can adjust the tightness of the belt by sliding the motor up or down.
It is VERY important NOT to run the machine with the belt too tight.
Having the belt too tight can cause the machine to run slowly and can damage or ruin the motor.
When the belt is adjusted correctly, retighten the screw that holds the motor on (remember to protect the paint on the arm of the machine from the screwdriver this time, too!).
You will be most successful adjusting the belt if you hold the motor in place while you are tightening the screw that holds the motor.