batavus is tricky. i spent a good year working on this one. if you actually get your bike to go 40 (high 30's) your low end will be aweful, ESPECIALLY if you are running a biturbo. If you run a 2 piece biturbo, run the original header pipe on the biturbo and fine tune your header length- that is where the difference is once you have milled the head, ported the cylinder and upgraded your carb (devin is the guy to go to). If you shorten the header to the point where you can't pedal anymore that is when you can get in the high 30's. You will hate riding it though. You are better off starting off with a longer header pipe and tuning it (slowly cutting it down) to where you easily hit 33 top and learn to live with that. You will at least be able to accelerate somewhat decently and not get run over when the light turns green.
If you are starting fresh, and have any experience welding i would figure out how to put a puch engine on that frame. a lot of people are figuring out how to do that and it surprisingly is probably the cheapest way to get a batavus going faster (solid 40s). no joke.