I think a Ryobi 31cc would be too big dimensionally for a 4* 1.20 especially the long shaft version. between the rear carb and the long shaft it would be really long and you would have to add a bunch of weight to the tail.

A friend of mine has a 4* 1.20 with an Evolution 26cc gasser and that is a nice fit. I think you would be much happier with a gas engine that has a front or side mounted carb in that plane.

there is some good info on the 31cc here http://jagengines.com/