If you change the mirror - you'll need to get one with the same focal length, so that your telescope tube, secondary, and focuser position all match up.
Or you could simply get the old mirror re-surfaced (that's what I did when I built my 10" reflector around good quality used mirror that I acquired)
Yoy don't give your location - so I'm sorry I can't sugest anywhere local for you (unless you are in UK)
I got my mirror re-coated by these guys...
(They also sell ready-made mirrors of various sizes) [Registered users can see links. ]