I am sure that I have used KX500 sleeves in my Tecates.which use the same as kx250s. If memory serves me right you have to use the 500 seal also do to a larger OD sleeve.
The biggest issue I have seen on them leaking is getting some tension on the Oring like was stated use shims under the snapring some of the aftermarket sprockets are thinner thus there is no tension held in on O ring