dude, if you're going with a freewheel, you DEFINATELY dont need to rebuild the rear (you actually dont even need to for a fixie, but many here debate that). Screw the current freewheel off; screw on an bmx freewheel. Youre done. viola.

You may need to add a spacer or change the ring from inside to out (or vice verse) for a perfect chainline, but that should be about it...