I've done this fix based on directions I found elsewhere. You've left out one vital detail, which is what kind of IR emitter to get
I've had my roomba at least 5 years and I've had to fix the bump sensor twice. There is an LED deep in the guts of your roomba that gradually dims, and when it goes you get roomba going in circles and beeping nine times. You can fix it but it requires some crazy disassembly and some soldering. See here for a guide: http://www.schneordesign.com/Avi/irobot/...
I have some extra LEDs from the last time I did this.