Getting a big bore piston and sleeve kit for your model cylinder determines if it is possible to modify your engine. I rely on L.A. Sleeve for kits for older models that use cast iron sleeves. However they don't list any kits in their catalog for your model bike. The earliest year RM125 thats possible to make a big bore is the 1990 model which was the first year for the plated cylinder. In that case we can just bore and replate the aluminum cylinder.
Thanks, Eric