Hello!

I want to replace the D15Z4 engine of my Honda Civic 96 and I'm having issues to find another compatible engine, can anyone suggest me what motor fits best in my car?

well i mean any d series motor will fit in the OEM spot, you may just have to make a few adjustments

What adjustment should it needs?

I readed that this engine can be swapped by non-Vtech ZC and D16A6, what would be better?

You may need to change the flywheel, cluch, dizzy, and a few sensers besides that all most all d series motors can be swaped around the d17 is the only one I know of that cant other wise the nexted step is a vtec d series witch will go in the same spot as your non-vtec just need a vtec ecu and run the 3 vtec wires or get the vtec motor wiring harness witch if your going to swap a motor any way that would be the way to go the vtec will give you more power at a higher rpm

Depends on your power demands and what u r willing to spend, simple swaps= d16y8, d16y7, Single Cam ZC, d16z6, none of these motors r going to make a real increase in power given the weight of our cars, Non Vtec ZC your best bet as far as power and ease of swap, try to get the engine harness and ecu on whatever you decide and it wont be anything u cant do yourself.