Since you are skilled in creating assets and levels, that part should not be too much of a problem for you.
Still, it is going to be a one man job, so you should probably expect to spend between a few weeks to a few months here, depending on the scope of your project (how many assets and how big and complex your levels are) and how fast you can actually work.
The scripting part, though, might be a little more tricky. If you do not know anything about coding, it is unlikely that you would be able to code AI on your own because that is far more advanced stuff.
You could utilize and modify the game's original enemy behavior scripts for starters but it would be a challenge as well if you cannot figure out what all the individual lines of codes and variables are and what they are supposed to do.
Since it is usually the programming, play testing and debugging that would take up the bulk of the production time for any projects, taking into consideration your lack of a background in programming, your estimated completion time for this segment is 'unknown'.
It could take between 5~6 amateurs working full time to complete a mod with 2 levels within 2 months.
So you should expect to be spending much more time than that working on your own.
They use c++ in Unreal 4. Other common languages for games development are Javascript and C#.
If you are really keen to create games or mods on your own, you should really consider picking up coding first.
Otherwise you should look for a programmer that you could collaborate with.