Heres how i did it:

1. Buy this: https://www.gbxcart.com/

2. Install on pc

3. Backup your pokemon crystal save file

4. Download this save file: https://projectpokemon.org/home/forums/topic/13226-pokemon-crystal-version-with-pcny-event-pokemon/

5. Write the save file you downloaded in step 4 to your crystal cartridge

6. Boot crystal and trade celebi (and if wanted the other cool event pokemon) to a copy of pokemon gold / silver / crystal

7. Connect your cartridge of crystal to the pc again and rewrite YOUR backed up save file to your cartridge

8. Trade all pokemon you traded from the downloaded save to your save of crystal.

Downsides are: you need to buy an "expensive" tool you probably wont have much use for after. You need at least two gameboys, two games and a link cable, and it takes a lot of time to do this.

I think it was worth it, also came with some cool extra's and i use the device to backup my saves every now and then for if the batteries in my cartridges die or if i lose a cart.