Depending on the year/generation, the WR and YZ motors are basically the same except for the e start.
Cyndi and I have owned a '98 RS125 since 2011 - it has been her only track bike since then, so we have some time with it. The biggest problem with it, is that she loves it, and doesn't like riding anything else. She's ridden a couple R3's and other 300's, but doesn't like them compared to the 125. Over the years I've done a couple complete rebuilds on it, and there's been a few cylinders and pistons as well, but to be honest, nothing crazy. With that said, she isn't racing this bike - it's purely a trackday bike, and she doesn't ride the piss out of it, despite my encouragement to go out there and completely rape the thing, she still takes it pretty easy. The few times I have ridden it, I do ride the piss out of it and have a blast doing it. If you cover the bases on maintenance, use quality fuel and oil, keep the thing jetted in the ball-park (again she's not racing it and doesn't need every ounce of power - I keep the jetting a bit on the conservative side), and are good with getting it warmed up well before riding and keeping an eye on temps, it will treat you right. Pics are Cyndi riding it, DiSalvo riding it, and me riding it....
You look look like me when I rode my RS: Like a monkey humping a football. Thing is, I didn't care cause it was so dang fun to ride! I wish I could fold myself onto one these days. I know I'd love it, but I think I'd be hobbling around for days after every weekend at the track.
Yup, they are a blast!!!!! I really am too big for the thing..... but I can't help myself to ride it sometimes....