Dennis, if you like your Hobbistar with the 1.20 stroke, you're not gonna be happy powering the GP Spacewalker with anything less than a 1.20. Unless you fly off tall grass, props like 15x8 or even 16x6 would be okay. If you have a .90 already, it'll fly the Spacewalker just fine. But if you're looking to buy an engine for it, you might consider a 1.20 4stroke. I had a Magnum 1.20 FS mounted on my Spacewalker briefly before switching to a Zenoah G23. It fit inside the cowl quite nicely with only a bit of the head sticking out pass the bottom of the cowl. Power- and weight-wise, I think a 1.20FS is ideal for this plane.