91 acura integra questions?

The car is a 91 integra gs. B18a1, 1.8L dohc, 1834cc. I can go on all day with specs. When I bought it the guy had put a nguyen? Exhaust on the car. Stainless steel. Looks like 2-2.5" from the exhaust manifold back. Has echo chamber after the cat. It had a blown head gasket. I took the head off, pressure check, resurfaced, valves and seals were good. Replaced the whole gasket set, arp head studs, bosh platinum+4 plugs, wires, new lifters, rockers, timing belt, I'm running royal purple oil and filter. When I was finished, I droved it for maybe 200 miles(not straight). When I went to have it inspected it failed horribly. So I took it to a shop and the guy said my 02 sensor isnot even working and he said the cat isn't getting hot enough. He thinks its just a resinator. That there's no catalyst inside. Just looks like a cat. It doesn't blow smoke when I'm idling but I was told it smokes when I am driving it. Any thoughts.