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You could write your command handlers directly into the Say2 packet. It's dirty but should work.

Get a clean vanilla system folder.
Get l2encdec.
Extract it into the system-folder.
Run patcher.exe.
Now you can edit the L2.ini
Client should start now.

(Don't forget that you probably need...

it seems as if the port 7777 is already used on your computer by some other program or l2 server

You can use the implemented Telnet-server for that.
Implement new commands and use for example PHP to connect to the telnet server and send the commands.

Guess you use Windows 7 x64 or Windows 10 x64.
You need a patched and unpacked system folder.

Dear Community.

I am looking for the old C1 Client of L2, but all links I found are dead and a torrent I found is incomplete.

Do you know a working source for the C1 Client or the other L2...