francofly, a quick check of the User Review section here at RCU revealed that one pilot used the 140-sized Super Tigre G2300 2-stroke on his and was happy with it for leisurely flying. Another pilot flew his with a GMS 1.20 2-stroke, and he mentioned that he was thinking about moving up to a 1.50 2-stroke instead.

Nobody was flying on a 4-stroke smaller than a Saito 1.80, however.

If your O.S. 120 happens to be the 1.20 AX 2-stroke, it will probably be a good match for leisurely sport flying. If instead you were thinking about using an O.S. 120 4-stroke, then you might need to rethink your engine options. There is no mention of anyone flying the Giant Super Sportster ARF with a 4-stroke that small.

I hope this information is useful. Good luck, and let us know what you decide to use for an engine.