The siren issue is an easy fix. Any reputable car audio shop or alarm fitter should be able to replace the siren with another universal battery back-up siren. I've done it for plenty of Brant owners whose sirens have fallen prey to high pressure cleaners (water ingression) lost keys, etc.
Here's an example of what you need -
http://www.dynamco.com.au/products_p...?model=SIR-001