I had a 2002 ford focus that somewhat frequently shot the spark plugs out. The last time it happened to me I decided to just drive home on it. I thought I was only misfiring on one cylinder but when I got home I looked at it and saw that the spark plug that shot out had pulled off the plug wire from another spark plug as well. So I was running on 2 cylinders for approximately 20 miles. Top speed I could hit was about 60 mph. Power was very lacking. It definitely runs rougher on 2 cylinders and doesn't have enough power. On 3 cylinders, when it happened previously, it actually wasn't horrible for power, I had no problem going 65, but it did run rougher than on 4. If it was done well a 3 cylinder conversion would probably work ok. 2 would really be pushing it.
I was just reading about this 2 cylinder engine that gets better mpg than my honda fit, and has only slightly less horsepower. 56 mpg and 105 HP.
Green Car Congress: Fiat introducing new 2-cylinder 0.9L Turbo TwinAir in 2014 MiTo mini