You can use a 1 1/8 fork in a tapered headtube. From the Salsa site about building up a Spearfish:
"For a straight steerer tube (1-1/8") fork you need a ZS44/28.6 | ZS56/30 spec headset. The key difference here is the "30". What this means is you use the same 56mm ZS lower cup and bearing, but then switch out the 1.5" crown race for a 1-1/8"
Cane Creek (as well as others) offer these headsets.
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