Novus (available at most Harley shops) has a nice 3 step solution for you but...
If it won't wipe off, you are going to have to remove material. That makes it bad in a couple ways.
The first is that most face shields use some method of hardening the outer layer of plastic. You can remove enough material to make it clear again, but you will be in softer plastic, so it will be much more likely to scratch again with less effort.
The second problem, is optics. Manually polishing and removing material is going to turn the optically correct shield into a (in)corrective lens, because it is impossible to remove the same amount of material through to entire surface of the shield. It might not seem like much from the outside, but after looking through it for any length of time, you can almost guarantee yourself a nice headache. Imagine having to drive through an old house window with all the waves and distortions.
My recommendation, live with the scratches (I have been for better than a year on one of my shields) or replace the shield. If you're going to replace it, get one with tear-off posts, and always run a couple layers of tear-offs. That way, on the next drop, clearing the shield of scratches involves removing only a tear-off or two.