Hey Beethoven, I did some searching and found the Electra Sparker 3i on the Mike's Bikes website here (I assume that's what you meant when you wrote Sparker i3?) it looks pretty similar to the Townie and other Electra bikes and yes, I think you could definitely convert it to be electric. In fact, back in 2008 I think the guys at Pedego were using Electra frames to make their original ebikes
Here are some options, you could use a pre-made kit like BionX or Falco which comes with the motor, battery pack and control console or you could get something a little bit less expensive like the Clean Republic Hill Topper that just has a throttle button, no pedal assist or fancy computers. Alternatively, you could get a wheel conversion kit like the Electron Wheel by Currie. This is the easiest solution but it adds weight to the front wheel and can change the ride.
There are other kits out there that I haven't reviewed yet and a couple more electric bike wheels (the FlyKly and Copenhagen Wheel) that aren't quite out yet. I hope this helps steer you in the right direction. Just make sure you get the right wheel size to fit your bike, it looks like it has 26" wheels which is pretty standard.
Yeah, the system looks pretty cool and they have been responsive with me as well. I'm heading to NYC around mid May and am hoping to take it out for a test ride! Fingers crossed, hopefully I'll get the chance and can report back