The Metro 1.0l engine is rated at 55hp, the Samurai motor is rated at 65. So unless you have an SJ-410 you will be taking a step back in power and torque. Unless you get a 1.0L Turbo, which is rated at 65hp but 89lbs torque. The same as a 16v, but you get that turbo wine!
No matter what you swap in mileage isn't going to be great. The issue becomes aerodynamics, your trying to shove a brick down the highway at 70 mph. It's going to create a lot of resistance, even if you don't weigh a lot.