Here's a few good links that will show you the "HOW TO" of doing this conversion if you want to try it yourself.

Sportster 883 to 1200 conversion (http://www.nightrider.com/biketech/xl883_to_1200_page00.htm)

2002 Sportster 883R 883cc Motor Converted to a 1,200cc - Hot Bike Magazine
(http://www.hotbikeweb.com/tech/0309hb_chopper_conversion/index.html)
r.m.h FAQ v3.9 (http://rmhfaq.com/hop_up_883.htm)