Stock alarm siren pretty quiet

i wouldnt replace it with a different siren or any type. the brain that controlls it is probably ment to run it specifically.  such as amperage draw and +/- triggers ect.  its not usually the best thing to do, mix and match factory and aftermarket.  you could check a similar vehicle of the same year and see how loud it sounds at.  if yours it very different sounding, id suggest a direct replacement or check the wiring for corrosion.