Alfred, you are going to get lots of recommendations for engines you aren't interested in when you start threads like this. The Saito club will chip in with their recommendations, the Nordic division of the Cheap and Cheerful Model Engines Enthusiasts Club will recommend the latest delights from mainland China and you might even get Motorman come in with a blanket recommendation to go with a YS 1.10 or 1.40.

So, to focus somewhat on the two engines you mention, this is what I know...

The OS AX 1.20 is a strong engine that is easy on fuel. Check out the pattern forum - a lot of guys there use the AX 1.20 in airframes like the Venus II and Excelleron as practice ships. They are very good engines. They are stronger than a YS 1.10, which is in turn stronger than an OS 1.20 Surpass, which is in turn stronger than your Magnum 1.20 FS. The AX would be a good choice for your expoits. It should pull that MA K series prop that your current engine does 8900 rpm with (I'm guessing it's a 15 x 8 or a 16 x 6) at something approaching mid to high nines - that's a big difference.

Good luck and let us know what you decide.

Edit: Prop size