Another option, if it's like the one on the C22 New Design, it to replace it with a small piece of polycarbonate plastic.
Just a note...
On the New Design, there are 2 pieces (upper and lower). If the upper one leaks, or is cracked, the lower piece will trap the water and likely allow it to seep into the hull coring.
When I replaced mine, I eliminated the lower piece and made a trim ring out of teak to cover the edge of the fiberglass.
Now, if/when the deadlight leaks, the water will simply drip into the cabin, which will make it more likely to be found and therefore re-sealed.