How to choose a fork for a swap

[arnehulstein]

Yes, I have too much time on my hands for thinking... Which means that I have been thinking about the XJ550 in the garage. I would need to replace the fork seals, but as I would be working on that, I might as well swap out the fork. How can I find out which forks fit? And if I would like to have a fork with more modern performance, but that could still fit the bike (preferrably with a straight swap without too much messing about), how do I go about selecting one? (Make, model, type, checking it etc.)