I dont run magnum engines so Im no expert on this subject but I would think the prop you are running is fine. Thats the same prop I run on my saito .72 for hovering (usually). It sounds like to me that you are a little bit underpowered as far as vertical goes. Knowing that you are running a 13-6, I dont think that a prop change is going to make a whole lot of difference. If you want to try a prop then go with a lighter, wider blade prop. I would say a 13-4w APC (these are about 15 bucks) or a 13-4 Zinger wood prop (around 5 bucks or so). I say the wood prop would be your best bet due to its light weight producing higher RPM with a Low pitch.
If you do decide to change engines I would go with either a Saito or a YS in the .90 range ( I think both are .91). YOu might also check out the new saito 1.00. I believe its lighter than the .91 and produces more power. When you check the price on these you will be stunned at first but you get what you pay for in this hobby. If you choose to go 2 stroke then I would go with something like the MVVS (it offers more bang for your buck IMO).