For the more common Kennedy "Hexacut" Model 60 hacksaw the flat belt is normally 1" (25 mm) wide and 21.8" long (554 mm) - however, because the size may have changed over the years, it would be safer to measure yours to get an exact figure. As the belts are custom-made, if you order the wrong length a refund is not possible. An accurate measurement can be taken around the pulleys with a strip of paper stretched around, overlapped, marked, and then carefully measured. Please put the length details in the order comments box. You might also want to turn the machine upside down and check the position of the adjustment bolts to see if the two pulleys have been moved apart to tension a stretched belt. If so, slacken the bolts and push the unit forward to get the minimum belt length; this will allow the maximum adjustment for stretch. The rarer and larger Model 90 "Hexacut" was fitted with a belt 1.25" (32 mm") wide and (32.5" 825.5 mm) long - though again it's best to measure yours carefully before ordering. Belts are made to an exact length and are £22 plus post and VAT. The belts are endless and made from a high-quality Swiss-made material designed for machine-tool drives.
The Belt for the Model 90 is normally 1.5" (38mm) wide - but appears to have varied in length. To to order one just go to this page  https://store.lathes.co.uk/belts/betalon-multiply-flat-belting and fill in the required details.
If in doubt about anything, rather than email, it's better to phone: 01298-871633 from 09:00 to 23:00 6 days.