No matter what you do, your FJ62 is still going to be a heavy vehicle with the aerodynamics of a brick.
The most eco-friendly thing you can do is use the money you would spend on an engine swap to do 2 things:
1: keep it running in good condition to keep it out of the land fill and prevent excessive fuel consumption.
2: get a fuel-efficient commuter car for when you don't need to haul anything or you're not going off-road.
If you insist on spending money on an engine swap, I would recommend a modern Toyota diesel such as a 12HD-T. They were never sold in the US, but are available from a few parts sources. You could potentially get ~27mpg or so, but be prepared to spend somewhere around $10K for the swap. Swapping in any other gasoline engine will likely cost a bit less, but it's virtually impossible to get more than 17mpg with real-world driving.
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