I have a GP Spacewalker and it started out with a Saito 100, great engine but not enough for ME in this plane. I switched over to an OS 120 and have never been happier. If your going to put in any engine smaller than a 120 you are going to have to add a bunch of nose weight, in my opinion that weight might as well be in the form of usable power.

I have 10 4-stroke engines Saito, OS and YS, everyone of them is a great engine as long as you put them in suitable airframes and are familiar with the operating differences of each engine. You can't go wrong with any of them, no matter what anyone says.