Testors finish

If the overspray is fresh and it's flat acrylic, then use some Windex to remove it. If it's lacquer you will need a new replacement as the lacquer thinner is too strong and will damage the clear part. As a last resort try applying Future to see if it neutralizes the flat finish.

That all depends. is it a car model or aircraft model? Is it a canopy? If so, pop it off your kit and try and clean it with thinner and polish again. Or order a new canopy from the manufacturer.

I'm making the assumption (and yes I understand what "assume" means) the canopy is firmly attached to the model,  In that case, I would mask around the canopy and then get some superfine automotive rubbing compound, a damp rag and proceed to rub the overspray off the canopy.  Finish the job by carefully repainting the frame of the canopy and then removing the tape.  Just a thought

New windows I'm afraid.