Hi Mike,
First, I'm jealous that you have a Skystar 90 already. Second, that they are cheaper over there - how much are they (in US dollars? maybe your local hobby shop would consider shipping here )
I have a G-Trick which first flew with an OS 91FX. I modified a wrap-around header made for the ST2300 (I just drilled and tapped one new hole). THe plane flew well and the engine ran great - very user friendly. However, curiosity got the better of me and I put one of my Hanno specials in it. I hate to say it, but the Hanno has more power. Enough to be noticable, but you can still fly all the maneuvers (I've flown all the FAI sequences) with either engine. I ran my 91 just with pipe pressure (no pump) and the tank close - it was very reliable. You might try the Hatori wrap around header made for the OS 1.60FX (I think you wil still need to drill and tap another hole). I run 10% nitro and my G-Trick weighs around 8.5lbs (3.8 kilos).
If you keep the weight at 8lbs (3.5kilos) or less it should fly great on any engine. By the way, does the kit include the fixed gear or did you have to purchase it separately?
Happy building!!!
-Will B