would you need the silencer for performance or is it just to keep it "silent"
Quite a few race kart engine builders claim they get a little more torque using that muffler (compared to an open header) on engines like yours. That muffler just has screens inside and its shape ads an extra stage to your header. It'd still be pretty loud with that muffler, just not obnoxiously loud where your neighbors would get pissed and call the cops and stuff.
Whether you use a muffler or not, you should buy or build your own support bracket for your header. (i prefer support brackets that clamp on at the end of the header or muffler)
If you call/email NR racing and ask:
http://www.nr-racing.com/ They'll sell you a genuine Honda 6.5 flywheel for around $20+shipping. They take them off brand new engines they build up using billet flywheels. The Honda Flywheels are supposed to be higher quality than the clones.