Check out www.mopedriders.com and click on "Moped Repair" at the top. Look for the 76-78 Batavus Repair Manual. It's good for pretty much every Batavus with a Laura M48 motor. As far as changing it.... Once you take the footrest and chainguard off, you should see two bolts, which go through the frame. They go through a metal piece which also attaches to the engine, its sort of a upside down U shape.
You should also have a set of bolts on the left side of the bike. Its the adjuster, and once you get to looking at it you can figure out how it works. Its a threaded bolt that rides up against a stop. By tightening the bolt you apply more pressure to the belt, by moving the whole engine forwards. When you loosen that bolt, you can move the engine back, enough to give slack on the belt to simply take it off. Read the manual it has pictures!