From memory the friction beearing in the extention housing is a non replaceable part and nissan recommends replacing the extention housing as the fix. Look up posts from 1200 forum member "Peter Zerket", as he had found a suitable friction bearing and method required to replace the bearing.

or alternatively find another extention housing for your gearbox with a bearing within service spec. being a 4sp, this should be very easy to locate. if you have no luck in ZA i have several housing in OZ you can have for the cost of postage.

cheers