I am a long time fan of the older Pokemon games and their distributions. I have just recently learned of all of the hard work of users in this community that have been working to restore these old distributions and their Pokemon to the masses. I have recently started a playthrough of the Gen 2 games and have been researching ways to get Celebi and am very happy to see that users have different save files that allow for the acquisition of legit Celebis. I have a few ideas and some things that I have been testing out, but I don't have the knowledge or the skill to do some of these things by myself. I would really appreciate it if someone could help lead me in the right direction on who to talk to for hopefully helping me bring the lost Western Celebi even to life without the use of a Gameshark. If anyone has information or would just like to chat about my ideas I would greatly appreciate it. Thank you.
If you wish to restore one of the Celebi on real hardware the most easy way to is backup / restore a save to a game cartridge using a USB card reader, one such way to do so here: GameBoy / GameBoy Colour USB Save Game / Rom Backup If you are using emulators then you can simply trade between two games using a supported emulator such as VBA-M or TGB Dual. On the 3DS the Virtual Console version of Crystal has the event unlocked to catch your own after you beat the Elite Four.
I am playing on real hardware, but I am hoping to take it a step further. From my understanding, Japan had a GS Ball event that you could use the Mobile Adapter with a phone to download the GS Ball to your save file and use that to get Celebi. From my limited research, this event is coded into the US version of the games, but an event was never made for it. I have seen online where people have been able to recreate this event in the US games through methods of altering ROM files. My idea is to be able to recreate this on an unaltered ROM cartridge by creating a custom mobile adapter that could allow the US games to connect to a hosted server and retrieve the necessary code to begin the GS Ball event, similar to how the mobile adapter worked for the Japanese version of the games. I know that it will be a huge project and may not even be possible, but it is something that I have been trying to mess with for a while now and seem to not be getting anywhere due to my limited skills and knowledge.
I have never used this, however have a look at: PicoAdapterGB - A Mobile Adapter GB hardware emulator based on RaspberryPi Pico I'm not sure if its reverse engineered to the point you can get the event, however it seems to be possible to do a Mobile Adaptor GB trade between a real GameBoy and an Emulator on the PC.