I've got a '93 DX hatch.
I'm swapping in a '99 Ls B18.
I got it from a guy who, supposedly, already converted all the wiring to go into HIS '93 DX, with the DX ECU, but changed his mind and went K swap.
It doesn't come with an ECU and I'm not sure I trust his wiring.
I still have my '93 DX engine harness, ECU and distributor, as well as a '95 Integra Ls ECU and distributor.

My questions are,
Can I use my '93 DX harness on the '99 Ls B18?
If I can use my DX harness, which ECU and distributor should I use?
Are there any other parts I should get from the '95 Ls?
Are there any other modifications I need to do to make this all work?

Thanks for reading this and any help would be greatly appreciated.

Yes you can. But your harness is OBD1 and the LS motor is OBD2. So you will need to swap some plugs from the LS harness to the Dx harness. I know that the injector plugs are different for one. Do some research on LS swaps for your 93. You may find that you can plug the Ls harness into the Civic's shock tower sub harness. As for the ECU get a obd1 P75 Ls ECU or get a Hondata S100 and chip and tune the Dx ECU. If you use a S100 any OBD1 ECU will work but it need to be tuned for the Ls motor. If you use the Dx harness use a B18a1 dizzy if you use the B18b1 harness use the B18b1 dizzy. As far as the rest you will need 99-00 Si rear motor mount bracket and 99-00 Si timing belt side motor mount bracket, use the Ls trans bracket to get the Ls motor to bolt up. You can use all your stock rubber mounts. You wil need a 99-00 Si or 94-01 Integra shift linkage.

yes you can use your dx harness. I would just swap to obd1 injectors instead of cutting and splicig the harness. Use you dx injectors, it's the same cc as the ls injector, 240cc. Use a obd1 ls distributor, or you can depin it repin it. Use a obd 1 ecu p75 94-95 integra or pr4 from a 92 and 93 integra.