I agree with members who said that 1KZ-TE is a good engine, however its good for mild to moderate off roading. For real tough terrain I would suggest 3.4L 13BT or 4.2L 1HZ, 4.2L 1HD-FTE, . Especially combined with low weight of an FJ40, the latter will give enourmous amounts of tourqe for any sort of terrain. However Its a pure technical comment and iam not aware of the cost or easy availability of those engines there (especially 1HD-FTE).
regarding 13BT, i have personal experience and i can recommend this motor. 2L and 2LT (2500cc) which came in LJ71 and Land Cruiser II are only recommended if u plan to take your jeep to Muree only, otherwise they are designed for light duty versions (LC II later renamed as Prado).