Giant OverDrive road fork replacement
You will need a crown race reducer.

1-1/2" -> 1-1/8" is a pretty common size. The 1-1/4" -> 1-1/8" that you are looking for is rarer but it probably exists. Maybe the de-evolution from Chris King.

If you can find that part, the existing bearing can stay.

Also do some research on the Problem Solvers headtube reducer; you would just use the bottom piece. If you use this, you can use the existing crown race (and bearing obviously).