This is a very very important decision to be taken. Basically i am of the opinion that it's never the bigger engine which helps in crossing any tough terrain but it's always a combination of different factors.
See these videos, all the vhicle running on a 2 stroke 439cc (just 28 bhp) and performing very fine. The first one is of my lj 50 with 660cc engine. So in the first stage, i will be just going for the extension, body work, 4 doors, a little wheel travel and will just run the existing 1000cc in it. Later if believed, will go either for the 1300cc or the diesel ones.
https://vimeo.com/93079675
https://www.youtube.com/watch?v=c9OijtrkW-U
https://www.youtube.com/watch?v=gfUfm4EtPPg
https://www.youtube.com/watch?v=0T6fxMwZ7CE