Do you only have the root parition or do you have everything else too (like /boot)?

1. If you only have root, reinstall the OS then just overwrite the root partition with your old one.
2. Run fsck on the root partition and/or use gparted to expand the ext4 partition. You might have to fuck around with the disk UUIDs to get it to boot again. You'll most likely have to at the very least change your /etc/fstab file to update the UUID of the boot partition.

if you have everything, you can just dd the sd disk to the emmc, provided that you didn't mess around with the partitions after.

1. Copy with dd to the emmc with or without the count, it doesn't matter that much if time isn't a priority. (something like dd if=/dev/sdcard of=/dev/newemmc). If it fails and says the device is out of memory after 32GB, that's fine. Any other error is not fine.
2. Use gparted to expand the root partition.
3. fsck the partition.