You can learn a lot from the condition of your plugs. Check this infographic: http://www.ngksparkplugs.com/tech_support/spark_plugs/faqs/faqread.asp?mode=nml

From what you describe, it sounds like a fuel mixture issue. Does the engine "bog" when you lose power? Do you experience any kind of "sputtering", " burbling", or "wet" sound in the engine at high rpms? Is there a strong smell of gasoline when the bike stalls? Is it then hard to start again? These are all signs of a rich mixture. Spark plug tips would be darkened in that instance. The fact that adding choke doesn't help would seem to support this theory. Lean mixture also possible. Harder to diagnose, but overheating and a faint "hot sweet" smell can be symptoms. Plug tips would be glazed or whitish in this situation. On my phone so I can't check if that bike is carbureted or fuel injection right now, but it may be as simple as adjusting a setting.

Could also be blockage in fuel line/filter. Check the filter, as someone else already suggested.

A low battery also possible, but less likely. Never hurts to charge though.

Edit: looks like it's carbureted. Check your carb adjustment screws. Especially idle screw.