Since Beachead are units, there are no others penalities from being "Striked" ?
Can I do a Sea Landing to a beachead with a Strike marker ?
Beachheads are actually markers, although they are, in some respects, treated like units. They're sort of in limbo. Honestly, the question never came up in all the hours of playtesting, and the aforementioned is probably why (markers aren't valid targets).
So, the real question is should they be valid targets?
I think they should be, however, I'd treat them as hardened targets. In addition, I'd allow them the same capacity as a port (2) which is reduced as described in the Alternate Rules of play (Strike 1 reduces it to a capacity of 1; Strike 2 reduces it to a capacity of 0). A Strike 2 result will flip the Beachhead to its No Supply side. A Beachhead with a Strike 1 marker which receives another Strike 1 or Strike 2 result receives a Strike 2 marker. A Beachhead with a Strike 2 marker already on it ignores Strike 1 results, but is removed on another Strike 2 or Destroyed result. A Beachhead which receives a Destroyed result is removed. When a Beachhead is removed in tnis manner, place it on tne Game Turn track two turns ahead of the current turn.