This is just my 2 cents, but if your bottom end is shot, your piston and con rod angles are probably all over the place, which would back up the wear around the ports, I would guess it is v-shaped tapering from the edges of the ports towards the center of the ports going south (towards the crankcase). Anyhow let's address what you stated in your question : Old rings replaced without cleaning the piston, buy a new set of rings to match your piston, take one of your old rings, break it in half, clean the grooves on your piston using the sharp edge of the broken ring, be carefuful not to remove any piston material. Take the piston off of the con rod,don't loose the needle bearings! Put the new piston ring into the bore, then use the piston to push the ring down into the bore so that it is square with the bore, measure from the piston skirt to the cylinder base, this needs to be spot on, lay a straight edge across the skirts, your measurements should be within .010 of an inch.Carefully pull piston out so as not to disturb the ring(s), then measure the end gap of the piston rings. Check against factory specs. If it ain't right, gap it until it is. Put piston back on con rod with bearings but no rings,oil it up, oil cylinder up, put cylinder back on motor as if you were reassembling it,pull stator cover off,slowly roll motor over until piston stops moving up, mark piston position with magic marker,roll engine over until piston stops moving down,mark piston position with magic marker. measure roughly in between these two marks, put piston head at this mark and measure the skirts with a feeler gauge. This will tell you if the piston is too small for the bore. As far as the wear on the exhaust side of the cylinder, you will need to take a snap gauge and mic and measure radially around the cylinder above and below the exhaust ports. But even so, if your bottom end is "shot" Your piston is going to be doing the "hula" whilst going up and down the bore. Hope this helps
