A compression test should show if your rings or valve guides are worn. Is there excessive blowby from the crank case breather or oil filler cap? If you do need to replace your pistons and rings, you will have to remove the engine, strip it, have the bores oversized and honed, and oversize pistons and rings to suit. While the engine is stripped you might as well replace the valve guides if they are worn, re seat your valves etc. It isnt a small job, and you will most likely find other bearings etc that are a little worn, and i usually feel inclined to get it all machined and get undersized bearings, so its all like new, not just one part. Either decide to keep it simple, or go all out.