Dont take my reply on this thread as a mean or sad one, but the "age of closed source" exe clients and servers are dead. Even for the open source projects are sad times when windows 10 does a update but can be fixed in a patch. And this comes from a admin that struggled with Holley version for 12years. Limitation of spells, monsters, its down to the patch that was back on the Korean that had back then.
I seen some chinese servers updated to korean with C++ but its hard and lots of experience is required.
On Holley/Kings - you got **** interface, placements done wrong, broken spells (DestructiveSurge/Blow bug, DragonAscension bug ... etc ).
On GSP might be worth to edit frames on dead and ****ed up spells that are around 5-6 if i remember right, and adapt damage,range,effect, frames to some Korean spells). I been searched gsp/holley/king for my project and things i discovered now that i wish i could discovered them back 5years ago.