It's very important for proper alignment.
That said...
Should you not be able to source a replacement dowel, or be in a bit of a time crunch, try the following:
- Drill a hole in one of the plates that hold the cam gear on, where the dowel sits in the sprocket and in the camshaft. Make sure the hole you drill is larger than the dowel.
- spread a thin layer of Red Loctite on the surface between the cam gear and the camshaft snout.
- Install the sprocket on the camshaft, the dowel for alignment, and the plate on top of it all, making sure the dowel is exposed by the hole you drilled in the plate.
- Torque the 3 bolts to the recommended spec.
- Using a magnet or other means, remove the dowel. Install the 2nd sprocket on the other camshaft using the dowel and the other unmodified plate as you normally would. Torque to recommended spec.
- Allow a few hours for the Loctite to set up, and drive as normal.
I've done this once for someone, and 3 years later all was well when the replacement dowel was installed.