Well you are a lucky guy to have one of their beaders. Looks like a nicely executed tool.
The fence is mia on my beader and I need to make one. The original was pretty small and I think the repro had one very close to the original. I have used a Stanley #66 beader with a short fence and never had a problem with it. Others find it too short and have made larger fences. I think it is all in how you use the tool. I like to work from reference surfaces, so when I use the beader I first put down a reference mark. Then I let the cutter follow the mark. To me, the fence is only a backup for the reference cut. It does lead to irregularities, but then, I am an irregular kind of guy. If I wanted it straight and machine precise I would have used a router.
All you have to do on the P&W beader is clamp a long straight stick between the fence and the cutter. A few 2" C clamps would probably work. At least that would be my starting place. Good luck.