The trick to getting the screws loosened is this:
The "screws" on the Humvee have heads on both ends.
But - each "screw" is actually 2 separate screws, a little tiny one that screws inside the thread of the other one, which is much longer. (The heads on the little one and the longer one are identical, though - so it looks like just one screw with a head on each end.)
In other words, to get it apart need 2 tiny eyeglass repair screwdrivers - put one in each end and twist in opposite directions. One clockwise and one counterclocwise. (You're unscrewing the little tiny screw out of the longer screw.)
So, once that's done, for both ends of the strap, you'll have 4 screws -the 2 long ones and the two little short ones that screw into them.
Then you've got to find a strap - try calling Timex's 800 customer service number.