For a track only SM you want the YZ. If you were gonna do a bunch of trail riding as well, and could only afford 1 bike, I'd go with the WR, do all the "free mods", & maybe even put the YZ exhaust cam in it. The starter would also be nice for relighting the bike after a get off in a SM race, but it makes the bike quite a bit heavier than the YZ.
If I was going to build a track only SM and had to go with Yamaha, I'd start with a WR. Put a hot cam (or at least a OEM YZ Cam) in it. You'll still need all your standard SM stuff, but when you are finished you'll have a 5-speed, electric-start race bike that only weighs 10 or so lbs more than the YZ.
Of course, the other route would be to start with a YZ and put a 5-speed in it. I'd imagine the YZ tranny is close ratio, which would actually be better, if you could get a 5th gear in it. You still wouldn't have the electric start though, which I'd take for the couple lbs any day.
Either way you'll want to put a better exhaust on 'er. If RAD checks this board, he may have some ideas about the tranny issue.
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