I had an OS .91FS on my chipmunk also. It pulled an Graupner 12.5x7 3 blade prop at 8800 rpm. This engine is at the moment giving me a headache because its high end is very bad. But this is another story. I replaced the .91 with a OS FT120II pulling an Graupner 16x8 at 8000 rpm. The biggest problem with this configuration was its weight. The vertical performance was a little bit better, but not much. Also a problem was that the prop was pulling so hard at idle, the plane was rolling away when i let go. At the moment, as an experiment, there is an OS 46 LA pulling it. Now i have to pull 300 gr. of lead because the engine is so light. The engine survived a really bad mid-air collision, so it was just for testing it in flight. Its vertical performace was bad, with very strong winds. The used prop was an APC 11x7 turning at 10000 rpm.
By the way, the cg is about 10 cm (4inch) from the leading edge.

Greetings,
Vince