I've never actually seen that airplane fly with a Saito 150. I have seen it with an OS FX 1.60 -- AWESOME, a Saito FA-180 -- NO COMPLAINTS, a Moki 180 -- IN ORBIT, and a Zenoah G-23 -- GOOD PERFORMANCE.

Based on my experience and taking expense into consideration, your best bet for a gas engine in this airplane is the new Zenoah G-26.

There are others out there that will blow the walls off the Zenoah with the same weight cost but also at a more significant wallet cost. Another concern you have is that with the stock landing gear you are constrained by your prop size. Even with a 16" prop you need to be carefull with your takeoffs and landings on grass. The G-26 recommends the 16" prop. With larger and/or more powerfull gas engines you need to look at larger props. You can move up to an 18" prop if you invest in the TnT replacement gear.