Hi, My son is 4 and rides a Cnoc 16. They are geared quite low to help kids get going and I guess it stops them from riding too fast also. But he has just started coaching at the local BMX track and would benefit from having a slightly harder gear on his bike to stop him spinning out so much. The current factory gearing is 25T (front) 14T (rear).

Does anyone know if you can easily swap the rear cog for a 12T or 13T? What replacement cog should I track down (size/brand)? I've tried to find information on the type and size cog I would need to track down with no luck. Islabikes say that they have designed bespoke hubs/wheels etc to make them light so I'm worried the cogs/freewheel body they use are non standard size. Hopefully I'm over-complicating things and any old 12T cog will fit on. Thanks for any help in advance.