Mini-engine positioning on bicycle ?

With the new petroleum price it's time to re-examine the 60's bicyle add-on engines ?
Very few of them then drove the existing chain.
Now with the well developed 'multispeed gearing', it seems obvious that the engine should use this existing mechanism.
The extra forces to be handled should not exceed 50% of a strong rider, to be able to use existing/stock machines. This gives a very small engine.
Where would you mount it ? Could it drive a 'wheel' on the opposite side to the existing chain crank. Or directly to the existing chain ?
It would have to be clutched and probably geared down. Perhaps a belt-drive to do both ?
I wish I had access to one of those 3D CAD facilities.
Ideas/criticism ?
Thanks,
== Chris Glur.
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