Mini Drill Chuck Key & Belt

Does anyone know how to identify the sizes of 'Jacobs'-type chuck keys? I have a japanese Sakai/Toyo mini drill with a 'Yukiwa 5675 EL 6.5mm
Chuck for which I need a key. I have a Jacobs K1 key which has a 4mm pin & 11 teeth. The pin fits OK but toothed part is not big enough - ie it needs more teeth! Any suggestions appreciated. Also, does anyone know where to source 5mm dia round section belting for the same drill. I have purchased the 6mm dia section belt from Chronos but it is too large to run in the root of the pulleys.
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