I am going to bet that you dont have catalytic converters or O2 sensors.
Do you even have a check engine light on the dash?
I had to Google your country to find out where you are. lol
Keep us posted on what you find.

 

I would start with the basics first. When is the last time you replaced your
fuel filter? have you checked the fuel pressure, or just drained the tank from the fuel rail?
Why don't you shoot us a photo of the engine so we can be clear what engine
and EFI system is installed since this is not NAS spec.
There should also be some sort of emissions decal on the radiator support
You should have an obd connector somewhere and you should have stored fault
codes.
Cheers..

 

He said something about a 3.9L, I wonder if he's got a distributor. Even so, there should be the box with the LED display. Is it under the seat on disco's like it is on the RRC?

 

Yes he said he had the 3.9, which has a dizzy.
It might have carbs, but I am betting it is fuel injected, no cats, no O2's.
It should have the code reader box under the passenger seat, but it could have been removed.
The 3.9L was used in ROW until the introduction of the DII in 1998 as a 1999 model year.
There was also a 4 cylinder double overhead cam engine for the DI in ROW.