You may have trouble finding an exact replacement size. The round PVC thru-hull readily available as they are common on new plastic boats. The oval shaped ones are generally bigger than your 1.5" x 3" size. (usual 2 x 6")
Basically a scupper is just a hole. You could use anything that finishes it off nicely and the core is sealed off from water intrusion.
If you can't find a matching size scupper, you could use a stainless rope hawser such as this one
http://www.reeltackle.com.au/index.p...104&vendorid=2 to give your boat a bit of bling.
Another option is cutting out a larger hole to fit what size is available. Like most boats, (including mine), your cockpit drainage is probably too small to meet ABYC standards. They call for sizing to allow drainage of a full cockpit full of water in 90 seconds.