Welcome

You might try this question on the diesel discussion board also and might get some suggestions on procuring the ECM. I doubt that anyone here knows anything about the Tata, and so probably cannot make recommendations about the success of the swap.

Most engine swaps with which I am familiar have focussed on engines that do not require a computer to function, as these are much easier to deal with. The ECMs often require sensors linked to coolant temperature, throttle position, vehicle speed and so forth, which can be a problem with a swap.

You will also need to decide whether you wish to keep the transmission already in the Tata, and fabricate an adaptor plate to mate it to the engine and starter motor, or to transplant a compatible Mercedes transmission also. Is the existing transmission strong enough for the powerplant you have in mind? Torque is often the killer of weak transmissions.

Good luck

Kevin

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'85 300SD