I had help from a friend with a MIG welder, but today i modded my stock batavus starflite hs by welding a techno boss performance pipe onto the existing starflite VA header, which i cut down to just above the bend.
performance has increased ENORMOUSLY, and according to my sketchy speedo i can now hit 37/38 on totally flat ground. I seem to have lost a tiny bit on low end (this bike was a dog to being with), but acceleration and incredibly smooth now, and it just goes so much faster than before.
The one tricky part about this is accommodating the right pedal, as the larger expansion chamber gets in the way. to do this, we sawed off the end of an extra crank and welded it to the existing one, which gave just enough clearance.
credit where credit is due: this technique was imparted to me by MA user john mahoney break>