I assume you scratched the original screw on UV filter, not the actual lens?
I don't see a replacement available yet from DJI. You could call them, but I would just get an ND or two since I never fly without one. Maybe you'd want it for night shots it, but even then I doubt a scratch would make much difference if you decide to forego replacing it. If you're careful where you take off and land (or hand launch and catch) you could even just fly without it.
As for cleaning, I use a rocket blower just like with all my other photo gear, or a soft lens cloth or microfiber if it is lint free.