I do not know about a deep scratch, but I have eliminated fine hairline cracks in old turn signal lenses by painting them with a generous brushing of liquid hobby cement.

Testor's liquid plastic cement is quite similar to acetone and welds plastic (styrene) by melting it - you can wick it into cracks with a paint brush.

when applied to small cracks, it heals and smoothes out the plastic - do not try to brush it several times as the brush marks will show - just give the cracks a single wet coat.

If your scratch is too deep to sand off, perhaps this process can soften the edges of the scratch so that it does not show as much.