So you are talking about the hydrolic ram that powers the top to go up?
I've never heard of anyone rebuilding one. I'm not saying it can't be done, i've just never heard of anyone having a leaking one and considering a rebuild. My guess is that you'll never find the proper parts to rebuild it... I have no idea where you would go to get the seals.
I would look for someone parting out a 'vert and buy a replacement. You could check the "trading post" sticky thread, the classifieds forum here on NICO, and
www.car-parts.com (a listing of salvage yards - search for convertible top and then call to ask for the rams).
Let me know what you find out if you do any more homework on rebuilding them. It might be possible, and would be a great write-up for the forum, if you are able to get parts and get it done. Heck, some Nissan warehouse somewhere might have a couple parts in stock???