Bicycle Tutor

Bits of my older commuter bike are beginning to give up (the fork mainly), It has a 1 1/4 in headset (used 36mm wrench to overhaul it).
I now have a spare 1 1/8 in threadless fork spare (from an upgrade to my MTB) complete with star nut and wish to attach this to the old commuter bike (i'd overhaul the old fork but it's a sealed unit so I can't, the bolts have been welded to the outer legs in the factory).
I know I need a new stem and spacers (getting one soon), but what else do I need to do? I suspect the headset will need replacing, but I don't know what to get.
Can someone please tell me exactly what I need to buy (sizing) and how to do the job?

Hi JonB,
Besides your new stem and spacers, you'll need a new 1 1/4 headset and an 1 1/4 to 1 1/8 headset reducer. You'll need special tools to remove the old headset cups and press in the new ones. Your LBS should be able to do that part for you. If you want to do everything yourself, Park just came out with a home mechanic headcup press which is more affordable and good for the occasional headset swap. You'll need the Park headcup remover as well. Hope that helps.
**Make sure it really is an 1 1/4 headset. Many 1 1/8 headsets use a 36mm locknut. If that's the case, you can just get a new threadless 1 1/8 headset and forget the reducer