I have had 4 different rossi's when they were king of q500. great motors, aim for a prop that will do about 15,800 rpm correctly needled, which should rise to 16,000-16,200 when quickly pinching and releasing the fuel line. 9x7.5 or so. Hope the muffler is better than the pop rivet things we had to use. the pop rivets were cheap aluminium ones, replace them with stainless steel ones and put twice as many in. The only problem we saw was the variation of power they would make, some had good fitting ABC some didn't and the tach would show that . They always had good throttle response and the plugs lasted forever.