It should also be embossed on the existing belt.
Someone might have put on a one-size-smaller belt to get the alternator to fit inside the cowl.... I had to do that with my 6A SamJames cowl....
It is a type A belt - 1/2” width. Sorry but don’t remember the length, which will vary depending upon which flywheel you have - 2 different sized pullies.
You can slice your belt, lay it down flat and measure in mm. All belt numbers have the length in mm in their part number. Round down, as your belt likely stretched a bit over all that use.