First thing is to not mount it at all. Any writes to the overwritten partition will corrupt your data.
Second thing: install system rescue cd to a live usb and boot it. Look into testdisk and photorec. It’s been a while since I’ve had to use these tools, but I believe testdisk can restore the partition and photorec can find files in a file system that has been deleted. I would try running photorec first to save the recovered files to an external hard disk, and then testdisk to try restoring them. But disclaimer: it’s been a while since I’ve had to do this, so my memory is foggy here.