I'll second that about using solvents on Plexiglass. While there are some benign solvents that "might" not create a problem on the plastic canopy, it's not worth the risk.
To take paint overspray off of anything, you first have to know what kind of paint the overspray is made of. If it's an uncatalyzed enamel, then there are some mild solvents that might cut the overspray and not hurt the canopy, but I still wouldn't take the risk. Just polish it with a plastic polish over and over and over and over... until it comes out shiny and clear.
If the overspray is a urethane, then just polishing it with lots of elbow work and a plastic cleaner/polish MADE for plexiglass is the way to go, but it will be a LOT more work than if the overspray is enamel. There is usually a mild abrasive "cleaner" made for Plexiglass that you would use until the overspray is gone, and that would be followed with a non-abrasive polish, again, MADE for plastic. Again, lots of hard work.
If the overspray is epoxy primer, and you've allowed it to cure for a few days, then you are in for some REAL work. You would probably have to hand sand the canopy with a 1200 grit "wet" sand paper to get rid of the overspray itself, and then you would change to 1500 grit wet sanding to get rid of the 1200 "scratches" (you don't really see individual scratches at that magnitude of grit), and then change to 2000 wet sand paper to get rid of the 1500 scratches. Plus, you have to pretend that you are working in a hospital environment and make sure that absolutely no dust or dirt (blowing dust from your driveway?) gets in the water or on the canopy. If that happens, even small pieces of dust will leave deep scratches on the canopy that will polish out, but will add lots of work to the process. Keep sloshing clean water over the area you're sanding. If a piece of grit gets between your sandpaper and the canopy, you'll feel it. Make sure you use a clean sanding sponge and not your bare hand to sand or you might actually end up sanding grooves into the canopy that will show up as ripples when looking through it later.
Finally, you use a very fine plastic polish with a mild abrasive to get rid of the scary looking fogginess you created by sanding the plexiglass, and then finish by using a non-abrasive polish to bring it back to like new condition.
You can use a machine polisher with a lamb's wool pad to polish the canopy with (keep it clean at all times), but be VERY careful to run it VERY slowly and do NOT let it heat up the plexiglass or you'll screw it up big time where it can't be fixed. If you've never used a machine polisher before, it would be safer to do the polishing by hand, but be sure as well to use "real" polishing cloths, not those red mechanics towels because using the wrong kind of fabric for polishing can cause damage as well.
Using either a nasty solvent, OR heating up the Plexiglass with a machine polisher too much will cause crazing where you can see lots of teeny little cracks within the Plexiglass. If you see that, you're done; get a new canopy.
Go slow, be careful, DON'T try to shortcut by using any solvents, and DON'T get overspray on your canopy ever again.
Vern
RV7-A