What is your budget? As doc said, it's all possible when throwing money on it. There are conversion kits around, I think it's ashcroft. The price in the UK is about 4k GBP plus fitting. The other option would be indeed to fit in a petrol engine, but you still need the Defender specific parts for the automatic lever, pedals etc. Not sure if that wouldn't be more expensive. Or you find one somewhere in Sharjah and fit it into your Defender. I would be careful with the Japan imports, they are know for a rather poor reliability.