I like the greatplanes spacewalker and wonder if anyone has flown it using a 120 fourstroke??

I've got the Surpass 91 with the pump that they recommend and it is really a great match for the plane. A 120 should also work good and weighing a bit more will require a bit less lead in the nose. Of course, your fuel consumption is going to also go up.

I have an O.S. 91 4-stroke in mine, seems to be a perfect fit, but I've heard of others using a 120 4-stroke with good results.