Not having tried a 1.8 Duratec in a Morgan, but have done so in their original intended place in various Fords, based on which I'd also say the Sigma has it. "Out of the box" power output isn't great but very tunable at reasonable cost and super reliable as well. Even as standard its a wonderfully zingy engine that loves to be driven hard and rewards with a power curve ideally suited to a sports car. Add an MX5 gearbox, lightweight body and thin wheels/tyres and you have close to motoring heaven. If the chassis is galvanised as well its win-win-win.
