There's your problem. It's a small clean airframe. 10x7 APC if you want it to scoot. If it doesn't, your Magnum isn't doing what it should and needs replacing with a TT Pro or O.S. AX, et al. You really don't need more engine than a 2 stroke .46 for this model to make it perform well both in terms of either outright speed or vertical transition. But if you must have more, any of the .46 cased .52 thru .56s should do the trick!