I got this Shoestring at an auction.  It is setup for electric and I suspect that it is rather old.  No battery's were included and I also suspect that they never got it to fly.  The prop is a Zinger 6x3.5 that is press fit on the motors shaft.  The battery connector is a 9v plug clip that has been cut in half and for a speed control it has an on/off switch.  I am debating putting in a brushless motor but my fundage is rather short and all my other motors have planes.  I have a 25amp brushed speed control from my failed attempt at R/C so All I need to know is what battery pack to use.  Any Ideas? Comments? Suggestions?    

If I were in your shoes, I guess I would try and get a handle on the kV of the motor. There are two ways to do that.

1) If you have a tach, cutout a disk and mount it on the motor shaft. Then I'd use a D size Alkaline Cell and see how many rpm it takes. The kV would be rpm/battery volts (you should check what the voltage is under load).

2) If you have a drill press and a voltmeter , chuck up the motor shaft in and measure the voltage coming out of the motor. You still need a tach to figure out the rpm, but kV=volts out/rpm.

Then I am guessing that 6" prop needs to turn somewhere over 10k to fly the plane. I would then claim the battery you need will be ~80% *kV * pack voltage.