Time or Money? Which have you got?
Money: Google ""COBRA" KIT CAR REAR END BODY" and you'll find a Craigslist San Diego for what you need for $300. Then pay about that to have it shipped to you.
Time: Reverse the shape of the right side to make a part to repair the left side.
Buy a gallon of polyester resin and several yards of 6 oz. fiberglass cloth. In the states you can get this at Wally World, Home Depot/Lowes or the local auto supply store. Also boating stores. Heavily wax your right quarter panel. Then lay up about three or four layers of cloth over the right quarter panel. Be sure to include the edge of the doorjamb so later you can index from that point back. Let it sit in the sun for a couple of days to harden up, then peel the new part off the body. It will be quite flexible. Clean the inside with wax remover.
Now here's the trick to reversing the shape: slice the new part into 1 inch wide strips vertically (like a vertical blind) and then reverse each strip as you fit it to the other side of the car. In other words, starting just behind the door jam, cut a 1 inch strip from the new part and fit it into the hole in the left side of the body next to the door jam. Then cut another strip from the new part and fit it up to the first strip you just attached. Keep cutting and attaching strips in this manner until you get to the back of the quarter panel and have filled the hole. Sure, the edge of each strip will be slightly out of whack with the next strip, but you don’t care because you are only using these strips as a base so you can then build up the outer surface with more layers of glass cloth. I would use a saber saw with a very fine toothed blade to cut the strips.
Naturally, before you start repairing that quarter you’re going to want to cut out all the damaged areas leaving straight edges that are easy to match up with new glass. Sand off the paint and the gelcoat too. Streetbeasts advertises that their bodies are extremely thick, so if yours is more than 3/16 inch thick, you can thin down the edges to less than that by grinding off the gelcoat and also grinding the inside, “feathering” the edges back a couple of inches.
Get some 5 minute epoxy glue. Now you can glue up the 1 inch strips to the inside of the body, one at a time so you can place each one a saw kerf apart from the next. The strips will be low (depressed) all over so that you can then build up the outer surface of the body with 6 or more layers of cloth, and some of those layers are going to carry over onto the feathered areas of the old body surfaces both inside and out, thus locking the repair into the existing body. Several layers of cloth can be added at one time until the repair is strong enough.
Now the real fun begins: the repaired area must be built up to the proper level and must be faired so all the body contours are correct with no waviness, and not only that but the new surface must be made of a material that expands and contracts the same as the original gelcoat on the rest of the body or the repair will “map” through your finished paint. You can’t just use Bondo.
If you were a glutton for punishment you could keep adding layers of fiberglass mat (not cloth) to get the surface high, and then after it hardened up you could grind it down and then eventually cover it with gelcoat. You may already know that grinding on fiberglass is about as nasty as it gets. Alternatively, you could use Duraglas (get it at the auto paint store) which is an extremely hard waterproof putty and is approximately as hard as gelcoat. It’s hard to grind and feather so you would need serious power tools to work it on a surface that large, BUT it’s still better than trying to work fiberglass. Also look up Vette Panel Adhesive made by Evercoat, the makers of Rage Gold, for use as a top coat instead of Rage/Bondo. You can order it in gallons.
After you get done you may wish you had bought a new body for about $2500US. They show up on Egay frequently.