I see, so you want to have a set number of properties in the .cfg file and update the file when one property changes?
How to index the file?
If the size of each property doesn't change then you could easily index the file with a table of offsets placed at the beginning of the file. When you want to update a property, read the table and go to the appropriate offset to make the change.
The file size is very small, it' size will be around 0.5 KB.
If the file isn't huge and the size of properties will change, your approach is probably the best for small files. Anything more will overcomplicate things if the speed gain isn't worth the extra code. This is usually true for the smaller files.