To me it sounds like the problems are all originating in the carburator. Chances are that either something isn't properly connected or properly functioning and your adjustments were subsequently done improperly.
When it was rebuilt, exactly what was replaced? Was the temp sensor that operates the choke replaced? Has it been checked to see if it's functioning properly?
How did you adjust the carb? There is a proper protocol and an improper protocol. Did you have instructions to adjust it to?
Have you hooked up a vacuum gage? Was the vacuum appropriate and stable?
have you checked the fuel pressure and regulator?
You may need new plugs, but they're not the cause of your problems. I'd bet lunch they're all fuel metering related. Be aware too that lean operation can manifest itself as hot chamber temperatures (which can cause pinging) as well as a low top speed. The power to go fast comes from the gas, and too little gas in the mix means insufficient power.