How deep is the scratch? If it is cosmetic only, go with Lurker's advice.
If it is deep enough to be uncomfortable when you grip the knife, you can make the following repair:
Get some epoxy adhesive from the hardware store. Most micarta is epoxy based, and even if it is phenolic based, the epoxy will stick to it.
Clean the surface of the handle with acetone and dry it carefully with a clean cloth. Don't get any oil from your hands on the part you are going to doctor.
Fill the scratch, (better to put a bit too much than not enough.)
Once the epoxy is dry you can sand it down to the original shape of the handle.
The result may look "Repaired", but the knife will again be comfortable to hold. Whether it looks "Repaired" or not will depend on your skill with the final finishing. (If I were doing it, it would look "repaired".)