ORIGINAL: Capt. Yossarian
It's conceivable that for some reason, it's not actually the G4500, but somehow he got the wrong manual? It has a big 90 on the motor it self. I'll try to get some pics by tommorrow, I'll take super close ups of the motor and the docs that came with the motor, which say it's a G 4500.
Dave
If there is a "90" stamped on the motor then you most likely have the .90 size 2-stroke SuperTigre. That would make a lot more sense and it is an appropriate engine for that plane.