The Focus S corners are rounder, the camera cut is different, the front sensors and back speakers are on different sides and the Focus S has a physical camera. So a tight fitting case will not work.
However, since the dimensions are similar (67x126x8 mm [Focus S] vs. 66x125x8 mm [Galaxy S II]) a flip style case that clips in on the sides may work.
I was easier than I thought. Just made a Hard Reset:
Samsung Focus will be returned to its original state with full data wipe.
Dial ##634# from the phone pad press the call button when the diagnostic screen appears type *2767*3855# to perform a full restore.