i would not recommend changing the hs50 port from D to O. i think a better approach is to just get rid of the cast flashings to make it flow better. less invasive and probably achieves the same end result. using epoxy on an air cooled motor, so close to the combustion chamber, just seems like a bad idea. i guess you could TIG aluminum fill it, but that's a lot of work. and the welding may (will) warp surfaces. meaning you may end up having to bore the cylinder. and to what end?