Several things you could try, glue a strip of abs behind, use ABS paste or even fibreglass resin and matting from behind the crack to stop it splitting further.
Per the CRL
•The soft belt proper is made of canvas, or material with a canvas covering. Size must be 2.75” (70 mm) to 3.25”(83 mm) wide and not wider than the plastic portion of the belt. The color is be white to off-white.
From the CRL
•The soft belt proper is made of canvas, or material with a canvas covering. Size must be 2.75” (70 mm) to 3.25”(83 mm) wide and not wider than the plastic portion of the belt. The color is be white to off-white.