I missed the beginnings of this thread, but cool Idea on making it froma jetski, I made mine with a riding mower and tranny and was told it would be too slow or suck, or all kinds of things, I've hit 50 mph and it goes like hell, yes I have to pick gears well because its 17hp, but I can tell you what I used to tension my belts that is easy to get and replace, a belt tensioner off a chevy v8 flat/serpentine belt. I added an arm to it to add a clutch pedals I would have some leverage, it works awesome, and if you run two v-belts I am pretty sure you need two tensioners so they can be individually adjusted (connection to the pedal) for stretch. one other thing I would add is belt guards to keep them on when you push in the clutch and have deep pocket pulleys so it doesn't pop off as fast. should be a wicked ride have fun and good luck!