If you can live without this other clear part you lost, you can buy a spare canopy from Squadron Models. They have a couple different ones for the Mosquito but I'm not sure if one of them is made for the Revell kit, so you might have to somehow try and find out if they would fit. And, like you mentioned, you could contact Revell and have them send another clear part sprue. Either way it's not the end of the world, if you already put a lot of work into the model it's worth doing it right and getting the proper replacement parts, and you will indeed be able to get them one way or another.