Resurfacing the top of the cylinder is difficult because in order to remove the scale and warpness you have to remove a significant amount of material that warrants remachining the o-ring grooves. Sometimes the damage is so bad that the cylinder needs to be stripped, TIG welded, bored, faced, and re-grooved. The head suffers the same degree of damage and most times they need to be replaced. On some 125s its possible to turndown the cylinder enough to repair the damage if the head is being converted from a domed piston to a flat top design.