The simplest conversion is the 2.5 Sherpa version of the Landie 2.5N/A, injector pump misses OS engine mount, I did this to a series 3 I owned. Forget the TD. 200tdi is a popular conversion, you have to squeeze in the intercooler etc though. A few have done the 200tdi and done away with the turbo etc reducing it to a NA. Td5s have even been fitted!
Another popular conversion is the Perkins TD out of a Montego, very good in SWB and cost peanuts, Dudleigh Engineering do the kits.
If you want to go Jap (yeugh, personal opinion!) there are several companies that do the conversion kits. Problem with this route is that the cost of adaptors, bodged exhausts etc adds up considerably.
Trawl some of the main Landie forums for a ton of advice on this.
The worst conversion historically was the York 6 cyl diesel in a Rangie, a guy asking questions about one a long time ago was asked had he intended to build a high speed tractor!