Have you tried doing without the centre speaker? De-select it on your amplifier and it will split the centre channel signal between the two main front speakers. I prefer this to using a separate centre as do a number of people.
If your not keen on that I'd agree replacing the driver is not cost effective. Trouble is decent centre speakers tend to be pricey for no real reason. Even used ones. But if you can find another Valdus centre on eBay or somewhere it shouldn't cost more than fifty quid.
Agree that there's no need to have matching speakers all round unless you are using the system for multi channel music as well as films. Even then the benefit is slight unless the mis-match is extreme.
Current Lash Up:
TEAC VRDS 701T > Sony TAE1000ESD > Krell KSA50S > JM Labs Focal Electra 926.