I'd still go for using Tesco 99 or Shell V power and 60:1 first. Then I'd tweak fuelling, then I'd look at engine parts.
It would have needed to have done a shed load of work to need a piston and rings don't you think? My Rev 3 is a 2007 and on all it's original stuff and is silky smooth on decent oil and SUL but rattles like hell on 95 un leaded or naff oil at the wrong ratio.