I had one with a YS .91FZ. That four stroke flew the plane with authority and sounded good doing it:

* [link=http://www.whitetyphoon.com/rc/gsp_spit_show_pass.mpg]Spit Show Pass[/link]
* [link=http://www.whitetyphoon.com/rc/gsp_spit_figure_8.mpg]Spit Figure 8[/link] - at half throttle

I believe the ST .90 will be enough. My plane ended up close to 12 lbs with air retracts.