Like striker said, I didn't do any of the server sided modifications, so if that is your goal then you can do it by yourself.
However, your project seems to have the goal of remaking the whole game.
Now there, I can help you out with a couple of simple things, the reason simply being because I actually work as a indie game developer and run a successful business doing just that.
To begin with, I'm not going to compare this project to a kalonline Private server, because it wont be, the goal of this project seems to be to remake the whole game. Models, textures, etc.
So simply put, I'm going to compare it to making a whole game from scratch essentially, because it's almost what it is.
So to begin with, you need to realize that the amount of work and time you need to put towards creating content for your game, even a simple game like KalOnline, is huge. The amount of models to make a game look decent, is very big.
MODELS
I usually say that, a bare minimum is around 200 models, to make a small area in a game look decent.
In a open world game like kal, it's still very necessary to keep variation, or people will get bored very fast.
Another factor is time, you need a lot of time to create the models and textures. A decent model usually takes around 3hours to make, depending on what the model is, the content for the latest title of my team (ONLY 3D models) will take around 6-7 months with 3 talented 3d artists working full time, and that does NOT include polishing.
Another factor is that you need planning, meaning concept art, etc. That needs a concept artist.
ANIMATIONS
Animations are extremely time consuming, in any game you have, animations are time consuming as duck, we are fortunate enough to have invested in a Motion Capture studio in our company, which saves us some time, we also use FaceFX. Which saves a LOT on facial animations, but regardless, you will always have to do some animations manually, because you can simply just not do it with motion capture good enough.
SOUND
Now, this is another interesting topic, because you need a lot of sound, because in the end, without sound, even the most awesome graphics can't make up for crappy sound.
This can be broken down to sound effects, vocals, and soundtrack to make it easier.
Soundtrack has to be made by a music composer, we have 3 of these.
A sound technician will help you set up sounds such as shooting, walking, and other various VERY important-to-get-right sound effects!
You also need vocals, in other words voice acting. This can be done with a headset, if you want it to sound like poop. Or you hire professional voice actors and put them in a sound booth! (RECOMMENDED)
As you can easily tell, this is a LOT of work. And one person alone cannot do it. Hopefully you figure that out before you start wasting your time (on something that you wont even be able to sell anyway).
The above headlines are but a fraction of the actual work that goes into a real game, I just don't have the time to go through everything, I just wanted to show you how much the mere basics are to get something like this off the ground, stuff I haven't mentioned include things like game designers, producers, directors, programmers, and many many more. That are indeed essential for a succesful product!
If you still decide to go through with this, I recommend to try to save on your time by using stuff like a MoCap studio and FaceFx and model packs to save time on animations and models.