You can remove the cylinder base gasket and use crankcase sealer to replace it. Permatex makes a product for sealing crankcase halves on motorcycle engines. I have had no problem with this method. As for returning the port timing, you can file the piston at a 45 degree angle, at the port location. Just stay above the top ring groove. Make this cut equal to the amount that you lowerd the cylinder. When hopping up Cr125 Honda shiftkart engines we used this method to test port window openings. Pistons are cheaper than cylinders. When we got to where we wanted we would then make the cut in the cylinder and use a piston that was not cut. The engine wont know the differance and for a one off it should be fine. On a piston port engine, the bottom of the piston on the intake side can be cut to restore intake timing also. As you increase compression you may have to use super unleaded fuel. I for, one, have never been able to break a Ryobi 31. The book mentioned earlier is the best one out there for understanding two stoke theory.
Good luck and let us here how you came out. motorhd