Can my headtube take a tapered fork?
'Tange Seiki' is just the headset manufacturer.

Reading this https://www.cannondaleexperts.com/Si-Mountain-Headsets_c_1569.html it seems that Cannondale SI Integrated headsets were used on Headshock and Lefty forks, so there's no reason why your bike would have that type. 

SI bearings are pressed directly into the frame so if you have separate bearing cups they are definitely straight 1 1/8.