Is the Sram 2 speed auto hub what you're referring too or are you talking about a Strumey Archer kick shift hub? If it's the Sram are you planning on making it a jackshaft transmission so that you won't need a shift kit? Then you could simply put a centrifugal clutch on the engines drive shaft. Ok took a look at the engine and it comes with a built in centrifugal clutch.
If you did the jackshaft transmission, you could put a 52t sprocket on the left side of the rear wheel. Once you got up to speed for a gear change it be like having a 38t sprocket on your bike.
An article giving information on use, adjustment and rebuilding of the SRAM Automatix hub.
www.sheldonbrown.com
Solid two speed coaster brake hub with funky automatic shift- the SRAM Automatix is reviewed including changing the shift point...
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