Did you clean or replace your points and set the timing?

First thought & probably wrong, is, that it "sheared off the flywheel key" on its "full throttle" rev out.
Secondly "rebuilt" is a "very well" used word with a wide ranging meaning.
I'm just having visions of you push starting then hanging off the side like a flag in the wind stuck on full throttle.
Wish i knew how to use emoticons or smileys.
Hope you get it on the road to recovery without too much more hassle.
"Flywheel Key"---!!

when I was 10 my best pal and I stole sisters moped and did the push start waving flag demo in front of a squad car!  

Check the head bolts make sure they’re not loose because crackling sounds sometimes means air escaping through the head gasket. Just put a socket on it and make sure it’s tight, spark plugs, etc..

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Ahh, i was thinking you'd had the flywheel off.
But still could be that.
Need the proper puller but if it's slipped & taken the key the nut is likely 'less than tight'.
This type below but there will be cheaper version out there for sure. It's the most common mag puller out there i think, fitting most other brands of bike with similar mags.
yambits.co.uk/dt3-flywheel-puller-p-24058.html

If it was mine I would clean the points and check the timing and a new gapped spark plug if it won’t start it’s probably something in the carb

Ok good did you set the timing? 

Your timing could be off if you’re getting kickback. Do you have a dial indicator? If not you can check the points gap should be in the service manual