Do you have a picture of the crack?

You could forcing epoxy down into the crack end maybe reinforce it on the outside with some fiberglass cloth. Another option would be to epoxy a piece of thin plywood over the existing firewall. You could cut the ply to fit the entire firewall or just an area slightly larger than the crack.

My biggest concerned though is how did it get cracked and what else is damaged? It sounds like the nose wheel took some impact and damaged the firewall. You might be better off getting deeper into it just in case there is damage you can't easily see.