Piper J3 Cub pilot reported the mageneto casing broke and detached from the engine during cruise resulting in loss of power and a successful off airport landing.

Narrative

Summary: Off airport landing following component failure. Mag casing broke and detached from engine.Enroute to destination airport. 110 mph. 500 feet AGL. Winds 3-5 out of NNW. 3/4 fuel. Pilot sole occupant. Visibility excellent. Rural area; mixed fields and woods.Loud bang and engine lost power. Approximate RPM 600 or less. No other attendant sound.Immediately turned to adjacent field; into the wind; within glide cone. Throttle non-responsive. Set up for the field at best glide (55 mph) and landed without incident on loose recently harvested surface (aircraft on 26-inch bush wheels). Nil power on landing and engine quit completely once throttle closed. Three-point uneventful landing on soft field.No damage to aircraft or occupant. Inspection of engine immediately after revealed detached right mag hanging on studs (Slick mags). Mag case appears to have failed near attach points; with a break 1-1.5 long on mag seat under attach point hardware. Once broken the mag detached from the accessory case.Incident took place approximately half mile from a private airstrip. Aircraft towed over to same on farm road and secured in hangar pending further investigation of major component failure by AP/IA who maintains same.