Let me boil it down for you guys that are lost.

A 96-00 civic hatchback has the cat in the HEADER area of the exhaust system.

Traditional car:
Exhaust =====Cat=Header=Engine
Ek:
Exhaust========(Header/Cat in one)=Engine

A dohc swap in an Ek bolts right in using the proper mounts.

The exhaust systems for the ek will usually bolt up to a dohc swap with a little tweaking of the hangers etc. (ls/b16/type-r & some gsrs)

Note: The dohc header only has 1 02 bung because dohc cars usually have a cat right after the header.

Hanging an 02 sensor in the engine bay worked for me when I used to run my JDM obd2 ecu in my ek with a factory ls swap.

I have also seen this trick work in a obd2 turbo sohc civic of my friends.

It may not be the best solution but it prevents a cel

IMO, get a obd2 to obd1 conv harness & a obd1 chipped out p28.
It will solve this issue & it will enhance your performance.