SKM was a line of karts, chassis, and many components back in the early year 2000’s, that are still available out there.
They were kind of like the Righetti Ridolfi line of chassis parts
Although the more I look at your pictures, it looks more and more like a KRT chassis.
Honestly for that era anything would fit
Just remove the caliper pistons and seals, fins out what size they are, and place an order on any of these vendors below. Please note that if the caliper cylinders are too badly pitted you may have to replace the whole thing, but if they’re ok, then a re seal job will do, don’t forget to do the master cylinder also, as those parts do fail about the same time. Hope that helps. FastFreddy