what things need to be changed and how much hassle is it to put a 4wd engine into a 2wd cossie??

i bought another engine to go in my 2wd out of a 4wd as the big end went on my car... matey who fixed it put the 4wd bottom part into my car and put my old 2wd head on top... but is it simple to put a 4wd on there? the cylinder head cracked after about 1500 miles so am lookin for another head (as per sheps thread in wanted!) but they seem to be getting scarce lol