If it’s just an installer, can’t you just create a simple batch script to do the job?
It needs to be a bit more flexible than that.
The game directory has variable contents and subdirectories. (custom content and mods)
Installation can be form or to a removable medium. (basically from any location to any location)
It needs to check if the source directory contains game files and if the destination exists.
Create a directory if needed.
Copy the whole, including subdirectories, to the new location.
[that mimics the functions of the original installer]
Optional features
(re)Install the base game without additional content and modifications.
Install the game in the new location with selected modifications.
(de)Install modifications to current installation
- these options require a good understanding of the game and mechanics. They are welcome, but can wait.