As said by others - better off getting your original one repaired, or the vehicle info cloned onto a working unit.
I've found the later GEMS boards will run a motronic engine, but the early ones won't, so as Rick says - best to try and swap like-for-like if you must swap the whole unit, as then you're likely to have some sucess.
As wammers says, if the immobiliser code in the engine ECU doesn't match what's in the BECM, then the later Thor petrol engines will just crank but not fire up. If you have a really early GEMS BECM, then even if the code is matched, and Motronic is selected, it will still only crank but not fire up as I'm guessing the Motronic immobiliser coding wasn't programmed in until closer to when they were due to be fitted.
Rick-the Pick here in the UK is definitely the person to sort it out for you. I have the tools and could sort it aswell, but a) I'm away on holiday, and b) he'll be able to give it a thorough check over/repair if necessary and sort the lockout issues all in one go, which is almost certainly the better option if you have very little in the way of LR support or garaged where you are.