"The last version doesn't display anymore PI1 format..."
XnView displays a lot of Atari ST formats - certainly including all Degas formats. The only format I found it cannot handle is Deluxe Paint IFF ST (but Amiga IFFs work fine). None of these formats are turned on by default, try this:
Start XnView and go to 'Tools - Options'
Expand the General setting
Put a tick into 'Use all formats available'
If you don't do this you don't get any of the Atari ST file formats supported. If you DO do this, you get loads of them including all Degas / Degas Elite, Neochrome, Spectrum 512, all Tiny variations, etc.