Snagger
Explorer
You can fit a Discovery/RRC 200 engine, but the fan will be in the wrong place and the manifolds and turbo will be very low, which may cause added work. You would also need to use the engine mounting brackets from the 12J on the Tdi block,, which i did when I converted my 12J powered 109 to Discovery 200Tdi. However, for some strange reason not yet understood by anyone I know, that results in significant engine vibration being transmitted to the chassis, even though the 200Tdi defenders used those very mountings and rubbers... The odd thing is that Series vehicles suffer very badly, 90s and 110 suffer quite badly, but factory fit Def enders are relatively OK (though not as smooth as the Discoverys). It might be worth considering using the Discovery mountings, including the chassis brackets, if you have the complete donor and don't mind the welding.
You would also need to be a little creative with the exhaust, since the turbo will be in the wrong position for a Defender, and the 12J's 2" pipe is too small and has a baffled muffler box, while the Tdi needs a 2.5" pipe and straight-through muffler. You should be able to modify a donor Discovery's front pipe and connect it to a Defender rear pipe.
The gear box will connect straight up, but you will need to work on the fuel and cooling systems. The radiator should be a simple swap, and you can just use the rad and intercooler from the donor. The rad includes an internal oil cooler (right hand side). The header tank would ideally be swapped for the donors, but it's not essential.
The fuel filter is a simpler affair with bigger pipes, and can just be copied from the donor. The throttle cable does not need changing.
With a little creativity, you can continue with your existing air filter and its mountings - you may just need some new large air hoses.
The Discovery/RRC fan sits very low in comparison to the 12Js, though a Defender 200Tdi engine uses a very similar timing case and the same water pump as the 12J , so would be in the same place, enabling you to continue using the viscous fan and probably the fan shroud from the 12J. I don't know if it's possible to fit the timing case of a 12J to a Discovery 200Tdi (the case will fit the block and the 12J sprockets would fit the crank and cam shafts, but I don't know if the injection pump will fit the casing in the right position or if the pump sprockets are interchangeable).
A Defender 200Tdi engine would be simpler to fit, but are usually much dearer.
A 300 Tdi is another option, but would certainly require new chassis mountings.
You would also need to be a little creative with the exhaust, since the turbo will be in the wrong position for a Defender, and the 12J's 2" pipe is too small and has a baffled muffler box, while the Tdi needs a 2.5" pipe and straight-through muffler. You should be able to modify a donor Discovery's front pipe and connect it to a Defender rear pipe.
The gear box will connect straight up, but you will need to work on the fuel and cooling systems. The radiator should be a simple swap, and you can just use the rad and intercooler from the donor. The rad includes an internal oil cooler (right hand side). The header tank would ideally be swapped for the donors, but it's not essential.
The fuel filter is a simpler affair with bigger pipes, and can just be copied from the donor. The throttle cable does not need changing.
With a little creativity, you can continue with your existing air filter and its mountings - you may just need some new large air hoses.
The Discovery/RRC fan sits very low in comparison to the 12Js, though a Defender 200Tdi engine uses a very similar timing case and the same water pump as the 12J , so would be in the same place, enabling you to continue using the viscous fan and probably the fan shroud from the 12J. I don't know if it's possible to fit the timing case of a 12J to a Discovery 200Tdi (the case will fit the block and the 12J sprockets would fit the crank and cam shafts, but I don't know if the injection pump will fit the casing in the right position or if the pump sprockets are interchangeable).
A Defender 200Tdi engine would be simpler to fit, but are usually much dearer.
A 300 Tdi is another option, but would certainly require new chassis mountings.