Here is just little informatiom:
More than likely, you have a 2.5 NA engine. It is very similar to a British 2.5 NA but is different enough to make sourcing parts challenging. The good news is a 200TDI engine will bolt up without modifying the engine bay. There is also a guy selling adapter parts to install a GM inline six cylinder engine. I think it is a good option but not one at the top of my list.
You also most likely have an LT85 gearbox which was used with V8 British built Land Rovers, although they are different versions and not interchangeable. Santana actually manufactured them and shipped them to Land Rover in Britain. The LT85 is a good gearbox but is kind of “tractorish” compared to an LT77 and an R380 is even better.
The 2.5NA in my Santana is very well worn. When I get to the project it will be replaced by a 200TDI and either an LT77 or R380.
I saw a Santana for sale a while back that had that conversion completed.
If you are considering such a conversion, my advice is to play the long game. Wait for a good priced 200TDI and gearbox to come up for sale. That is what I did and I now have the 200TDI, an LT77, and R380 standing by. Many are pulling then from 90s and 110s for LS and Cummins swaps. They come up for sale often. I was able to get very low mileage units at very reasonable prices.
Oh, you also mentioned you want to change fluids. The oil filter in my 1988 Santana is an odd ball. However, I was able to find it in stock from a supplier in Illinois. It is used on some industrial engine that is common in the US. I have a spare if you can’t find one as soon as you need it. Just let me know.