Dirt Rider Magazine

I have a 125cc baja and it is in great condition other than the one missing piece which is the rear sprocket assembly that attaches to the hub. The specs that i need for the sprocket and hub is a 52mm inner diameter, 42 tooth sprocket. and my rims are 16's. Is there any compatible parts to it? i've searched high and low all over the internet and i need to find this part! I can find the sprocket but the assembly to go with it is nearly impossible to find.