paul
i started reading this and it has all the symptoms of a 2 stroke seize, then read XT, not sure about who makes the pistons
but there are cheapo chinese offerings for XT,s around, started searching ebay and came across this now that has a 4 stroke barrel and a mitaka piston the only one i have found though, the guys quote of 57 mm seemed possible but everywhere i looked the dates where wrong so i could not reply too that
Anyhow lets see what happens
valve seals are generally at the top of the valve stem under the spring while you have it torn down it would pay to replace these as they can make it smoke and you should have em in a gasket set