I don't know anything about the other kit you have listed but I do have the GEBE kit. I have the EHO35 4-stroke engine. I can tell you that has plenty of power. I weigh 250 and it will hold me at 26-27 mph. Since a 2-Stroke has more power I imagine the Tanka 40cc would really fly.

I'll admit the GEBE is a bit pricey. Is it worth it? I don't know. I guess it depends on how you want to use your bike. It looks like the BEC setup would probably have a fair amount of resistance should you wish to pedal w/o the motor. If you plan to ride your bike w/o the motor running you may want to take this into consideration. I can tell you that the GEBE kit is very easy to install and remove (except perhaps the spoke ring). The components seem very durable.

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We like our bike.
It is made for three.
Our Mike
sits up in back,
you see.

We like our Mike
and this is why:
Mike does all the work
when the hills get high.