Nick since I know you'll always be looking for the most possible high revs let me give you some little information, but please keep in mind that I don't know if the measurements are exactly the same to fit your engine.
The Kwaki EL250B from the technical infos should be the same like our 2-fiddy, with one difference, it is an air-cooled, but bore and stroke look the same and the engine should rev higher, so this engine is using double-valve-springs (you could search in the bay for "kawasaki el 250 zylinderkopf").
Good luck for your next steps
