@ALEXCARLSON Thanks for reaching out!
Unfortunately, we do not have the option for purchasing just the top portion of the Tarn 40 backpack. Additionally, I am not aware of any aftermarket options for that section either. After looking at the buckles on the pack, it looks like they are 3/4" dual adjust buckles. You could purchase 4 of the Gear Aid Dual Adjust Buckles in the 3/4" size and fashion a bag for the top of the pack that could attach using the buckles sides still on the Tarn 40 pack.
I hope this helps, thanks!