If you still have access to the M1 Mac:
On the M1 Mac, run brew bundle dump and copy the resulting Brewfile to your Intel Mac
On the Intel Mac
Remove /opt/homebrew
Install brew using the instructions on brew.sh
Run brew bundle install --file /path/to/Brewfile to reinstall packages
If you don't have access to your M1 Mac any longer, run ls /opt/homebrew/Cellar on the new Mac to get a list of installed packages, followed by step 2.1 and 2.2 from above. Then manually install the packages again.