The mid-range jawns are the LeoVince and the Proma Circuit. They're tough to come by, unfortunately, and you'll be losing a kickstand with the Leo unless you do some modding.
Proma GP is another good one. It's a decent all-around pipe, and has pretty conservative expansion chamber dimensions that provide good backpressure for a 50cc setup. Plus, it doesn't look near as ugly as a Biturbo.