The airline transport pilot was conducting a 14 CFR Part 91 personal cross-country flight when she smelled smoke in the cockpit. She flew to a small, rough airstrip in the area, while assessing the problem. Unable to isolate the problem, she elected to make a precautionary landing. During the landing the airplane encountered soft terrain and nosed over. Postcrash inspection of the airplane by an aircraft mechanic discovered that the engine starter had failed to disengage, resulting in an overheated starter and smoke in the cockpit.