If the replacement motherboard is the same make and model, you should be able to reactivate. If not, use the "activate by phone" method:
1.
Click on the Start button and in the Start Search box type in: slui.exe 3
and click on the icon that appears.
2.
Type in your Windows 7 25-character product key.
3.
Follow the instructions for phone activation (no charge).
Carey Frisch