It's actually a super simple task once you understand the format of Final Fantasy VII files. You can open up the char.lgp and find the files you want for Sephiroth's field model and rename them to Cloud's corresponding files and copy and paste them over. You'd have to do this for the bigger hrc model which I believe is AAAA.hrc in the char.lgp, but do not trust me on that one, that is me pulling up a distant memory. And you'd also have to replace the smaller p models for each of his arms, each of his legs, his head, each of his feet, his torso and if I am not mistaken his hands as well.
The worldmap model, if you wanted to replace that as well is located in the lgp archive respective to your country of origin. If you're in the United States of America, it would be world_us.lgp I believe, and I do not know what Cloud's model within that would be off the top of my head. You could either search through that with kimera to find the right model, or there is a file repository somewhere on this forum that has labelled and archived most models and files within the lgp archives very clearly. Which is a useful tool in its own right!
((Edit: I didn't read the first post carefully enough. I just realized you already have the world map model. ^^))
But, that doesn't account for Cloud's wheel chair model, parachute model, and any others that exist. A mod of this nature would require a tool called Kimera, if you wanted to manipulate an existing model of Sephiroth into Cloud's wheelchair/ parachute/ sword wielding forms.
I haven't been on the modding scene in some time, but it seems like it would be easy enough to do. Even for a beginner.
. You'd just need an LGP unpacker, there are few listed here on the forums. You might have to sink a bit of time and reading into this to fully grasp what it is you'd have to do, but in the end it is really the best bet to go. The modders on this site are busy with their own projects and are very quite friendly if you show a thirst for learning, but as Tenko said above it is unlikely that they would simply give it to you.
Your best bet for getting exactly what you want, is doing it yourself, in your own way! I wish you luck, and I hope you can make something wonderful if you so choose. I hope this post was somewhat useful at least and halfway enlightening. I have an awful habit of rambling on and ambling aimlessly when attempting to explain things clearly!
((Also, I swear a mod of this nature exists in full scale somewhere. It is a very popular idea. I am not sure if that exists still though. I haven't done any sort of search.))
