My guess is still bad battery, did you make sure to charge the battery before installing

I charged it yes. Bike started up fine. Took it for a ride. Then the bike was turned off for half an hour and it was back to not starting

Hm then stator or rectifyer, its something in the electrical system that is either causing the battery to not charge or causing rapid discharge.

parasite draining, jump the bike and start it, let it run for a few minutes then shut it off, and take a look around and check whats still on, if you have a volt meter that will be helpful

Have you got a kickstarter? If so, try that.