Saw a 125 pulled apart ~30 years ago. If I remember it right, the studs run through the jug into the crankcase. Remove the tank and exhaust pipe to give yourself some room. After the head is off, pull the jug ´up´. I dont remember if the studs had to be removed. rotating the crank so the piston is at BDC should lower it as much as the studs, Yamaha has a site with exploded parts at shop.yamaha-motorcycleparts.com/