When I started I had an OS 46FX on a .40 size Hobbico SuperStar Trainer, and never felt it was overpowered. In fact, I have never thought that any plane was overpowered, since you can control the engine in the same way that you can control the control surfaces - you don't have to use full deflection during normal flight, but it sure is noce to have more (power or elevator deflection) if you need it.

Since the Magnum and the OS 46FX probably weight about the same thing, go for it! By the way, whenh I indicated that overpowering is a good thing I am not talking about putting a .91 on a .40 size plane, due to weight and other structure related issues. Just sue some good judgement, including that of your instructors. If you are trusting your instructor to teach you how to fly, why not listen to what he/she says about other aspects of the hobby - at least until you have enough experience to be able to pick and choose what suggestions to use and which ones to discard?

Another "By the way" - If you have the .46FX on the plane you may well like the flight characteristics of the trainer enough that you won't be too anxious to move on to another plane/engine combo. In this way you can have a lot more fun with the 'trainer' than just learning the basic of flying. Most trainers are discarded too soon by new pilots, when they are quite capable of some basic aerobatice and therefore will be a lot of fun as you learn more about flying.

Since I seem to be bucking 'conventional wisdom' of some, I might as well go deeper...I disagree with those that tell you and others that thrust measurement on the ground is meaningless. I know that it is a valuable tool for making initial comparisons and decisions, while flying is the ultimate tool to fine tune those decisions. When making your thrust comparisons, just make sure that you are making good comparisons by having the plane on a solid and level surface, and that you have eliminated as much friction of the tires, wheels, axles, etc.