I have the Shoestring with a Thunder Tiger Pro 61 with a pitts muff and it has plenty of power! Yes, more will fit! I know you speed Monsters want to get that 100 MPH speeds. Mine is very fast with the 61. I played with the props to get a little more out of it... To say that it is under powered with the 61 is not true... It flies fine with athourity, with plenty of power. I never put a gun to mine but I must get 80 mph out of it. When I fly at the field I'm as fast as anyone. We do not have races at our field, but we do play. The shoestring is an excellent plane and flys great... I have mine set a touch nose heavy so it does land HOT! Once it touches down on the grass it stops like it has airbrakes. A MUST DO!!! Reenforce the landing gear area... If you do not it will break! I fly off grass and took the wheel pants off and put on bigger tires.
Have Fun