//writeini file.ini Some Section $replace(Some Item,$chr(32),$chr(95)) Some Data
[Some Section]
Some_Item=Some Data
There's also another solution that gets talked about alot on here. But is always argued against because it only works for certain fonts.
//writeini file.ini Some Section $replace(Some Item,$chr(32),$chr(160)) Some Data
[Some Section]
Some Item=Some Data
I'm not going to go through all the fonts to see which one works but mIRC's default Fixedsys will work.

-Andy