I did this upgrade over a year or so ago (you may be able to find posts from then), and I could never satisfy my self as to the rating. In the end I got a 100amp fuse based on the logic that that was what my alternator was rated for and thus I wanted to be able to use all 100amp, but that if the system was “asking” for any more I wanted the fuse to go. The stock link was likely rated based on what the stock alt was rated for or lower based on the highest draw state for the stock equipment. So if you want to have access to the full capability of your equipment for lighting/winches etc. then the rating of the original link is really the wrong question. The fusible link is there to protect against a dead short anywhere in the system and if you have one the 100amp fuse will let go.
Just as a note, there are now breakers available that can handle 100amp that would be nice, but if you use a fuse you need one that has a slow response, because other wise your fan motor turning off at the same time as the lights are on etc. may trip the fuse. Another thing; I left the old wire/fusible link from the alt to the power point hanging from the loom just in case I need to go back to the old system to get me home. I hope that was helpful. I love my new system everything works better; it even fixed my voodoo intermittent a/c problem.