Hi Pilou,

this is something what my STLimport script can do...
After importing a STL it has some functions to clean faces, polygone lines, curves and circles.

Select the object -> start the script and choose "Only clean"
It will ask you to "clean flat surfaces? click "no"
Then it asks: Redraw polylines? click "yes"
Select all polylines from the hole -> click "Done"
It shows you the first Polyline and ask you what it shoud be -> choose "Circle"
The same with the next one.

It delets all corespondending faces and lets you alone to rebuild it...

Have fun