A Lockheed Lodestar carrying 4000 lb (1814 kg) of marijuana suffered a no. 2 engine no. 9 exhaust valve failure. The pilot did not feather the propeller and tried to land at Pompano Beach Airport, FL. The aircraft stalled and crashed while turning on approach. PROBABLE CAUSE(S): powerplant – engine structure: cylinder assembly miscellaneous acts,conditions – material failure pilot in command – failed to follow approved procedures,directives,etc. miscellaneous acts,conditions – improper emergency procedures pilot in command – failed to obtain/maintain flying speed FACTOR(S) pilot in command – exercised poor judgment miscellaneous acts,conditions – improperly loaded aircraft-weight-and/or c.g. pilot in command – improper in-flight decisions or planning weather – low ceiling weather – obstructions to vision complete power loss – complete engine failure/flameout-1 engine weather briefing – unknown/not reported weather forecast – unknown/not reported emergency circumstances – forced landing on airport/seaplane base/helipt.