I thought I had a base gasket leak in mine for a while as oil was pooling in the dish behind the cylinder. Be absolutely sure it's that though, as you have to pull the cylinder sleeve off (up at the very least) to fix it. To do this you have to drop the engine which gets expensive if you're not doing it yourself.
Just make sure the oil leak isn't coming from the cam belt drive pulley seal. That's where my leak was actually coming from. Sounds bizarre, but somehow the oil was leaking out and tracking up to the back of the motor. It truly looked like it was the base gasket which was leaking. I got lucky and needed new belts at the time anyway, so was able to replace that gasket at minimal additional cost which fixed the oil leak. Had I gone and fixed the leak I thought I originally had, spent all that money, and then found out it wasn't the source of the leak, I'd be pissed.
I guess my point is that there are a variety of sources for that possible scenario (crank breather, side case seals, etc), start with the cheap and easier ones if you aren't 100% positive.
That being said, whether you are doing the work yourself or not, if you're going to the trouble and expense of dropping the motor and fixing the seal, why not take the opportunity to put in some HC pistons or something to that effect. It's not a lot of extra time/money, and it's not everyday that the average rider gets the "excuse" to pull off the top end of the motor to tinker...
Just my three-and-a-half cents
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