jackson,
Your last post may make a difference in the answers here.

Is this "mount" two wooden rails that are actually a part of the fuselage construction? Do they go through the firewall? (It sounds like it.)

If this is the case, then I would apply epoxy to the "Crack", and maybe wrap it tightly with nylon thread while the epoxy is still wet.

Wooden rail type mounts that go through the "firewall" generally mean that there isn't any substantial firewall, just a forward former; and that won't take the stresses induced by a motor mounted to it.

I think we need more information, and a picture sure would help.
Dennis-