Is it possible to replace a fusebox lever without replacing the whole box?
No, that box can absolutely NOT be fixed. There are parts that are physically broken that are not replaceable, aside from that fact that it is generally in poor shape. Plus, I see evidence that that right-hand line side screw has overheated.

Even if it could, as Aloysius said, it is really not worth it. Someone, probably a home inspector, will see it and make a stink over it being band-aid fixed.