Slightly changed, I am Cook's bark on the sea
James Cook's ship was the HMS Endeavour, a.k.a. the HM Bark Endeavour. (Note that you have to change the spelling for this result)
I can, but I am, as targets may be.
You can endeavor to do something. Likewise, an endeavor, itself, is a target, or goal.
Upon the air, I'm part of change,
Endeavor Air is a subsidiary of Delta ($\Delta$) Airlines
Referencing the Space Shuttle Endeavour, the final space shuttle built (A reasonable interpretation, but not the intent)
though my name, itself, has had a range.
Previously called Express Airlines and Pinnacle Airlines.
My children, they number quite evenly,
Except in the times where you may be.
Endeavor has 8 letters, but in British and older spellings, is spelt 'Endeavour' which has 9. You have to add a
you'U' to get that spelling.
Repeating my start, but once before
The start of Endeavor is 'E'. It repeats, once.
I end with the end of forever.
Its end is the letter 'R'