Originally Posted by dan_vector

The Novarossi factory appears to have closed down so finding anything Novarossi now will be very difficult. The best pipe/manifold for that engine is the NR 2015 pipe with the 41032 manifold.
The OS XZ-21 GT is a budget engine along the lines of the Novarossi ISON line - doesn't compare with the Mito GT5 which is closer to the OS R21.
To be honest now the best 3 engines for GT (still in production) would be 1. Ielasi Tuned O.S. GP5; 2. Maxpower GT5; 3. ONE O.S. GT5