Are you going to use the crank trigger set up?
I'm building me a 8hp GX240 Honda alter right now and was gonna use the ARC racing billet flywheel and a MWSC 14lbs front pulley and should bring me around 18ish pounds of rotating assembly. If that's not enough I'll have a 8" 20 lbs flywheel and machine it for a browning taper hub to put on my PTO.