Just wondering if theres anyone out there that actually restores plastic items... I have a set of tail lights for my 71 demon that need a little help.
I just don't know if theres things that can be fixed such as hairline cracks and scratches in the lenses...
Any help is appreciated!

Lenses are typically made of acrylic, invisible repairs are basically impossible. Plastic repair in general is quite labor intensive/expensive, usually finding NOS or nice used replacements will cost you less with better results.

Light scuffs and scratches can be polished using todays plastic headlight polishing kits or similar.

There is no way to fix hairline cracks.

Minor (and I do mean minor) scratches can be polished out as Scott said

Any acrylic crack CAN be fixed with carefully applied methyl chloride. This and an applicator can be purchased from plastic supply places. For cracks you would want the thinnest available so it flows into the crack. Once it sets/evaporates you can and will need to polish the surface.

Methyl chloride is what is used to fuse all acrylic - display cases and stuff like that. It is NOT a glue - it melts and fuses the plastic.

Done carefully and buffed out you'd never know the crack was there.

https://www.industrialplasticsnyconline.com/ProductDetails.asp?ProductCode=Acrylic+Solvent