The OS .18 is a side exhaust engine so the manifold off you CEN engine will be fine. As far as a muffler goes, it kind of depends on how you drive. If you want more acceleration than top end you need to get one that will boost the torque, alternatively if you drag race you need to find one that will boost the revs. OS make their own pipes which are apparently very good, personally I have a H.A.R.D. shark low end pipe and have header matched the manifold to give the top end a boost also. Hope this helps.