Scooterotica

Agree with the stator ^

Just a thought but could the new crank be slightly longer/shorter altering the position of the flywheel in relation to the pick up ? can't say I've heard it happen but could be an option

forget all the complicated theories; first check the health of the ignition spark with somthing like this

a bright blue spark can jump a 7mm gap or more on a healthy ignition at kick speeds

i like this one from eBay https://www.ebay.co.uk/itm/191896610534 from the states expensive
Cheaper prices from around £3 upwards on eBay for cheap spark testers a great tool for diagnosing a good spark quickly at the road side

muppet

As does pulling the cap and holding it away - if you can bear the risk of getting a belt.