The shape of the frame rules out a bottom bracket crank motor, so the only practical option is a rear hub motor which can combine with the derailleur gears.
That can be mated to a rear mounted carrier battery,
You can get the whole package for that from
Cyclezee, who market the very powerful eZee motor that's ideally suited to that heavy load.
An alternative is the Chinese BPM-CST motor, and I'm sure others will have further suggestions.
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