There are a couple of key questions you need to ask yourself:
Are you comfortable tuning your car, or paying someone to do so? If yes, MegaSquirt (MS) is an option. If no, you MAY be able to MS if you stay stock and someone has a solid map.
Are Carbs legal for your car where you live? If yes, this could be an option. If no, well... no.
Is there some other reason you want the stock ECU? That's up to you, but they are outdated, getting old, and finicky compared to MS. They ARE more plug and play (questionable in your case without a harness).
Sit down and think through your real goals and options, and do it right the first time.