Err, you just click on my name....but in case that don't work it's Reeperette@Juno.com
There's actually quite a few depending on model and engine...which I think is a Batavus, fer most of em, and you have three different "versions"...20mph, 25mph, and 30mph....and the two lower ones have restrictions you can work around.
One dude on this forum has an awesome Batavus page, with many mods that will work on a Trac, and you can rig up Biturbo (his page shows ya how) or Polini performance exhausts up to it.
There's a couple other minor things, but your best bet is to start with Brians page and then work offa that.
Here's the page info - http://www.geocities.com/miniengine