Thanks Felix, you saved me flaming you.
If you want a stroker get an early TZ250, they are built for the track, bits are easy to get and they are easy to maintain yourself. Pistons and rings will do 500 miles easy on trackdays, cranks will do 1500. A top end rebuild will cost £140 for pistons, rings, little ends, and pins, takes about 45 minutes to do if you aren't in a hurry-lots quicker if you are. They crash very well, don't break unless you hit them VERY hard, and will show most 600's the way round. My TZ has been known to show R1's and Mille 'R's the way round, and I'm an old duffer. The Honda RS250 is another good one but there are not so many early ones about. My TZ cost me £2000 3 years ago complete with spare wheels and loads of bits-oh and I'm not selling.
John