Anyone aware of exhaust yet or how I can get a bit more torque from my son's z90? I've removed the speed limiter, backed out the throttle screw, and vented the air box, but the little turd still won't make it up the smallest of dune hills. My son only weighs 48 lbs and isn't afraid to stay in the gas. I even added some little Sand Gear paddles (to no avail). It seems to do ok on dirt but falls short of fun in sand. My son had the oportunity to ride a Raptor 80 with an FMF exhaust and it smokes his 90. Help!