You can get an idea of the belt size by measuring the width of the groove of the pulleys.
To get an idea of the belt section,look at my post at 16:39 on 29-10-21 on the the Chester 920 Cross Slide Backlash thread.
At a guess, I would expect the section to be A or Z.
You can estimate the belt length by measuring around the drive and driven pulleys, for a particular ratio.
This will give you the outside length of the belt, so you will need to decrease that dimension a little, since belts are graded by their INSIDE length.
So the belt needed may be designated as A26, signifying an A section belt with an internal length of 26 inches.
Having got some figures, go to your local belt / bearing stockist and see what they can offer in toothed Vee belts.
Hopefully they will let you take a belt on a Sale or Return basis (unused of course ) to check that you have the correct belt. You need to select the shortest belt that will fit, to allow for any stretch.
It would useful, if you report back on what belt you find is suitable, as an aid for anyone else.
HTH
Howard