Klaxon/ texecom (merged 2010) no longer manufacture your sounder, have a look at either one of their C, K, or V ranges.
We use a lot of their signaling equipment includeng my fav the master blaster :),
You could drop klaxon signals a sales email but unless you find someone with the activert gathering dust under a counter then I think you out of luck.
Filling the hole's length left by the activert with a replacement is your challenge.
You say it looks shabby, me tell you something now which may be frowned upon but hey times are hard,
Polycarb discolours just like upvvc as it's open to all the elements,(clean it) they come up a real treat with T cut and a bit of elbow grease then a polish :D
e manno, April 2013