What you are after is to use the stock transmission? I would not do a carbon copy of what took place in the bangla workshops. Dirt and grime will find its way into the crank over time...and that one video had some sort of broken metal, forgot if it was the piston skirt or was it maybe the cylinder wall that was chopped off, imagine where it some of it ended up? In the crank likely. Both of those used an open cam chain, that is bound to cause issues?
Personally, I would loose the entire engine and clutch and gearbox. Go for the right electric motor that cover your needs in terms of power, torque and price. Run reduction gear and sprockets and chain to turn the rear wheel. There are so many motors and controllers to choose from these days I can't really see why one would chose to go the bangla route.