Thanks for the kind comments.
I used PTGui to allign the images. It is a payware program, but there are freeware programs available, that should also work fine for this purpose.
In total I had 37 images to allign. As the program is normally used for photographs, the tricky part is to tell it what lens you were using (having set FSX to 40% zoom, I got good results when saying it was a 28mm wide angle lens).
Then PTGui aligned the images just fine (I only had to set a few contact points manually - points that show the same location in an image and the image next to it).
This is what I got:
After that it was only selecting the projection mode (little planet) and fiddeling a bit, what should be the center of the image and direction.
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