The Civic S is a 1500cc 12-valve engine. The DX has the 1300cc 12-valve engine. In the US, both have the CVCC head configuration with a down draft carburator. Here in Canada, we had the European non-CVCC head that had around 4-5 more HP. Tuners from the US would buy up the Canadian heads and install them on the US cars.
The blocks are mainly the same. Only the crank, rods and pistons make up the differences between the 1.3L & the 1.5L engines. The cam is better in the Canadian 1.5L head too. The 1500S had a slightly better suspension package, including a trick torsion type bar in the rear axle. It also had better gear ratios too.
There are many tuning secrets to these engines that I have now purged from my memory (I need more ram).
One of the weak links on these cars are the front torsion bars. They will get seezed in there. You folks from the south might not have that problem. Also the rear frame cross-member and panhard bar attachment point tend to rust away, thus sending the car to the crusher. But, overall a decent car. I raced a few of them for years.