I've been told this is a terrible idea and just wanted greater confidence even though the warning against has been taken. I want to be able to fine tune a 1990 car with an OBD-I port in addition to adding features such as launch control. Understanding this I'd need to, at the very least, be able to use OBD-II. Is there a way to do this or am I a fish out of water?
I think it's a very laudable enterprise.
I also think your "fish" isn't gonna last long. Your best bet is to convert to an aftermarket injection and ignition system (Haltech comes to mind; but there are many others.)
OBD II is all about emissions compliance. If you really want to explore the extremes, it's gonna be about your tune. You need to gather all your inputs and end up with an injection and timing map that truly represents your goals. That isn't going to happen in OBD I, and you will cripple your creativity trying to eek it out in OBD II. The "OBD" level isn't the point. Can you flash it? Can you change it? THAT will be the point.
An OBD I car ECU is not gonna have the speed and the chops to do what [I think] you are asking. There may be nothing wrong with the block, cams, and cylinders. But computers triple in capability every couple of years, and it's quite likely an "OBD I" ECU can't even be flashed or reprogrammed effectively.
A modern aftermarket ECU injection system would give you the benefit of the latest sensor and computer capability. While expensive, it pales in comparison to trying to make ... "your grandmother learn to program C++."
(No disrespect - your Grandmother could be a genius. Send me her number...)
[cough ... Sorry] To answer your question, your best bet is the latest aftermarket technology that can be applied to any set of reciprocating pistons.
Keep in mind the journey will be very steep an fraught with many perils. Also keep in mind that with dedication you will own tuning and understanding about AFR and IC engines.
To me, that in itself makes it worthwhile. Best of luck.
As explained in other answers, the short of it is, no it is not easily feasible. For your purpose, a tunable after-market ECU would be necessary. You may be interested in an open-source ECU, that you can build yourself, provided you have some electronics and programming knowledge. It's been ported to 40 different vehicles so far, and you can add yours to the list.