I don’t believe you can use your existing top headset cup, the bottom you can get a Hope reducer for – which slides (read hammer, and file down for ages) onto the fork.
The best option would be a new headset, have a look at the Hope Pick and Mix. If it’s a 44mm OD, then you have loads of options – I believe Nukeproof do a cheap one too. Get your specs of you frame’s headtube, and then have a look what fits.
I wonder if any of you STW guys can help me out with a little issue iv had. I have a 1.5 inch headtube frame with a 1.5 inch headset fitted, i have just swapped my 1.5 inch steerer fork for another with a 1 1/8 inch steerer, I have looked at reducer headsets and reducer crown races, what do you guys suggest i use? Ideally i would use the current headset. Thanks in advance.