Thanks for the background info ...
There are big problems affecting my ability to say much about the situation ...
The US Domestic Market (USDM) Tacoma was a totally new vehicle from the ground up, introduced in 1995.5. It was a completely different species from the earlier Toyota pickups (aka "Pre-Tacos"), which were more similar to the Hilux models marketed elsewhere in the world.
It wouldn't be until the introduction of the completely new 2005+ model Tacomas that Toyota once again displayed some common features between the USDM Tacoma and other markets' Hilux models.
Here's my point ... USDM 1995.5 - 2004 Tacos were a totally different species from both the PreTacos that *might* share some features with a 1985 Hilux as well as any / all Hilux models sold elsewhere in the world in 2003. I have no idea whether anything I could offer based on USDM Tacos has any relevance to a non-US Hilux with the same engine.
Additionally, there's the fact your '85 with a 3RZ is a completely custom rig. This means there's who-knows-how-much DIY "frankensteining" that's been done to it. There's no guarantee that a frankensteined vehicle is built / wired / works like either or all the OEM vehicles from which its components were merged.
Having said that ...
IMHO your #1
priority is to check the engine codes. This means you need to figure out what you can connect and which OBD format is used.
If the connector in the engine bay was a legacy item from the '85, it's probably an OBDI (OBD-1) format.
If it was transplanted with the engine and ECM it's probably an OBDII (OBD-2) format. My *guess* is that this is the situation with yours.
On USDM Tacomas there were 2 OBDII connectors - one in the engine bay and the main one underneath the dash left of centerline (around the area of the driver's right knee). These two connectors used different plugs, but both allowed you to check codes.
I have no idea whether non-US Hilux models had the OBDII connector in the cab / under the dash, so I have no idea whether you're stuck with the engine bay connector alone.
It will probably be a hassle to figure this out and finally get to read some codes, but it's the fastest way to make the most progress on diagnosing the problem(s).
The fact that your rear O2 sensor on the exhaust is blocked off / missing suggests the ECM was basing its fuel / ignition manipulations on the front (engine bay) O2 or AFR sensor alone. If it had been a USDM Tacoma, it would also indicate some mod was done to fool the ECM into ignoring the absence of a rear O2 sensor input.
This strikes me as setting up a single-point failure possibility (only one O2 or A/FR input) that might explain your problem - i.e., your front sensor is bad or dead. Whatever front sensor is being used, this is one of the things I'd suspect and check first.
This is only a guess, and because of all the differences I have no idea whether it's even a reasonable guess.
Years ago there was an Australian Toyota forum site that had tons of info on engine swaps and other customizations done in your neck of the woods. If that site still exists, I'd recommend you visit it. Unfortunately, I don't remember its name, and all my records were lost in a residence fire some years ago.
That's about all I can offer. Sorry about that, but we're really talking different versions of different species here.
The only thing I'm certain about is that you need to figure out how to read the codes. You'll be shooting in the dark, and probably wasting time, until you can do that.
Good luck with it ...