In reply to Baz P:
You need a 1.5" crown race, assuming an appropriate one is available. Depending on the headset you might need a whole new rather than just a crown race (assuming of course that the frame has a suitable head tube size).
The new fork has a '1 1/8 to 1.5 tapered' steerer tube, generally just referred to as 'tapered', which means it's 1 1/8th at the top where the stem clamps but at the crow race it's the equivalent of a 1.5" steerer. In this case 'tapered' is a specific size of steerer.
You can also get 1 1/8th straight, 1 1/4 straight and 1.5 straight, with the last two being very rare (I think there are some other odd sizes which are even rarer).
1 1/8th straight is an old (in mtb terms) standard and tapered is pretty much the norm now except on lower end bikes. 1 1/8th straight will be what your old fork has.
The size refers to the diameter at the top race/stem clamping area. All steerers, even those referred to as 'straight' flare out slightly where the crown race fits. It's a tight interference fit so if it was the same size for the whole length you'd struggle to get the crown race on or off.
To add further complication there are even more headtube standards, with lots of headset sizes to provide compatibility with the various different steerers and head tubes. If you're already aware of these then fair enough, but if not you'll need to check whether you can actually fit the new fork and whether a new crown race is all that's needed. Crown races are specific to a particular model of headset and aren't a generic interchangeable part. Apologies if I'm telling you stuff you already know, if you're not sure the easiest thing would be to take some pictures of the headset or perhaps just go to the bike shop.
