That will depend on which eMac you have. An older eMac can boot from OS 9, and it's just a normal OS 9 install. If your eMac is newer (anything higher than 1 Ghz processor, and some of those as well), then you can copy a good OS 9 system folder from another Mac, and that should work fine. You would only have access to OS 9 in the Classic environment on the newer eMacs.
Can you tell us why you need OS 9?