During taxi to takeoff the crew sensed an odor similar to an electrical fire, and smoke became apparent in the cockpit. Both generators and the battery master were secured. The right generator would not accept a load and smoke was coming from the right wing accessory J-box in the right wheel well. Three occupants deplaned and initially extinguished the fire, which reignited due to hot metal and compromised fuel/oil lines. All systems were secured and the pilot then evacuated the airplane, which was consumed by the fire. The pilot estimated that fire fighting equipment did not arrive for 20-25 minutes after the fire began. FAA inspectors were unable to determine the fire source from the burned wreckage. However, the operator's report stated that the right starter solenoid did not open after engine start causing the starter to remain linked to the electrical system. This resulted in a system overload and subsequent electrical fire. Both fuel and oil lines were routed through the right wheel well for cockpit indications.