[Arran]

So what was wrong with the original speaker? Cone torn up? If there is that much difference in DC resistance it sounds like the 450 ohm job might be for an AC/DC set or some oddity like a DC mains or 6 volt vibrator radio. The lower resistance could result in higher B+ voltage but more importantly it will not filter as well. If you absolutely must use this speaker with the 450 ohm filed coil you should add a filter choke in series with it to make up for the difference.
Regards
Arran