Simple question will the blaster 200 crank fit into my 1978 dt175 crankcases and fit my flywheel etc? Cheers Rob

Simple answer - NO

You can probably install a complete blaster engine though.

Shame, in England finding a blaster engine is not easy. And there doesn't appear to be big bore kits for the 175. Thanks for your input.
 

A stock blaster has about the same HP as your DT175. 

A good port job from someone that knows what they are doing can make a huge difference...  I built a mildly modified 175 that exceeded 90 mph (GPS verified) on a dry lake bed.  Front wheel comes up under acceleration if you're not careful.  And there are many shops that know a hell of a lot more about modifying porting than I do.  (I also used a 77-79 IT175 cylinder head with centered combustion chamber and smaller volume to make up for lost compression after raising the exhaust port)

Stock exhaust can also be replaced with aftermarket...  someday maybe I'll get a "Big One" pipe from the UK if they are still available.