Cycling & bikes - Motorised Mountain Bike

Hello, and apologies for asking for your help. I'm ignorant about
engineering.

All the motorised mountain bikes I can find (e.g. on the Internet) bypass
the existing gears, and use fairly crude transmission systems such as
friction or belt. This is understandable if the bicycle is fitted with a
derailleur gears, which don't like being changed whilst power is being
applied; but there are other types of gears (such as the Sturmey Archer)
which, I'm told, tolerate it. I'd like to remove the pedals from an
existing bicycle and fit a small engine with a centifugal or similar clutch
around the bottom bracket driving the existing chain, add throttle, fuel
tank and exhaust. The finished weight must be below 35 kg / 77 lb.

I'd be very grateful for any comments or advice.

With thanks for your attention,

David

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