You might want to talk with my friends on this one since they have also accomplished solid hybrid bike designs and can help guide you on your options as well as provide pretty much any thing you need as far as parts whether its a BLDC motor and controls to sprocket and gear design
I bought my very first diesel engine years ago from them and they helped me with my initial hybrid designs so you would be in good hands that will also be cost effective!
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Taking the roar out of the crowd: Hybrid Motorcycles | Recreation content from Machine Design
Good luck,
GH..