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The terrible gear grinding sound is probably the starter system - if the battery has not been freshly charged it will not have enough power to run the starter relay/starter solenoid/starter motor/etc.
If the bike has not started then the exhaust pipes are not hot enough to ignite unburned fuel; so the backfire could be fuel igniting in the combustion chamber when the exhaust port is open - which could be a timing problem. Just thinking out loud here. Is it a points system? Perhaps, if so, a little attention there might free things up. Just guessing.
Did you fully charge the battery yet? Note that it is not good enough to simply charge it overnight. Some of these huge batteries may take 48 hours to come up to full charge. Best is to either do a specific gravity test [liquid filled battery] or use an automatic charger [with a green light].
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