To fight it in Court, you would have to show due diligence; in other words that you made reasonable efforts to check on your plate prior to operating your motor vehicle. The outcome at trial would be very dependent on how long your plate had been damaged for and how it had been damaged in the first place. For instance, it it had rusted through months ago, you'd have a hard time arguing you didn't notice or hadn't had a chance to replace it. If you were driving home and a piece of roadway hit your debris, you could reasonably argue you were unaware and/or hadn't had the opportunity to deal with it.
I would suggest replacing the plate right away. If you approach the Crown, show that everything is taken care of, there is a very good chance they would at least offer you a reduced fine. They may even offer to withdraw the charge if you can present a reasonable explanation as to why it was damaged. Just be aware even if they reduce the fine, you'd still have the conviction on your record. It's not a serious offence, but some insurance companies look at all convictions.