Small diesel pickups are out there in the USA, but they are rare. Probably the best are the Toyotas but they are underpowered for most people in the US. I would find them so too as I think my gas 4 cyl. Toy pickup is underpowered and it has 20 percent more power than the turbo diesel Toy pickups.
There are some people who are putting the 5 cyl MB 1980s diesels in some Toyota pickups - supposed to be a good swap and a long life engine.
A few stick in a Cummins 4BT but that seems to me to be a very heavy engine for the truck, limited RPMs (makes its power between 1600 and 2000 RPM) and would need a close ratio six speed built to handle the engine, which the only ones suitable would add hundreds of more pounds to the already heavy combo. Then you would have to add axles that could handle the weight and power. It would probably wind up being a 4000 pound truck - which while lighter than my almost 8000 pound Dodge with a 6BT in it, would probably be an altogether different pickup with regards to handling.