Alex....changing out the belt is a serious task, something I rarely do unless absolutely necessary. Also I find it very unusual that the valve would be your problem (certainly not impossible) but typically when you gear up the speed and have hydro oil fly all over it is the variable speed sheave leaking, not the valve. The oil will leak out of the sheave onto the belt and the belt throws it everywhere often soaking the valve and giving the appearance of a valve leak....you're there, I'm not, but just be sure. If it should turn out to be a 'sheave' leak....then I would change the belt, otherwise....it would have too be one REALLY bad belt before I'd under take that job. Clutches are fairly easy....pull tires, pull cover and there they are.