The Magnum is an excellent airplane, I think, especially if it your first large model. I powered mine with an ST.51 that was pretty much stock. This engine makes good power as it is, and for a little extra punch, get a Tom Lay rework. You shouldn't need any more than 6 ouinces of fuel for this engine, and that makes it easier to fit and plumb a tank. Pay attention to the control set up, and I would agree to the before ementioned Tom Morris system. Don't use the kit stuff if it is the generation with the 3/32 wire and nylon horns. I would also agree with the mount for fuselage mounted gear, in addition to the wing mounted gear, and change to it when flying over grass. I just think it looks better with the wing gear. Try to keep it light, around 60 ounces at least. I got carried away with the decorations on mine and it ballooned up to 70 or 72 ounces, but still managaed to fly fairly well. I had several 500 plus point scores with that airplane. I used my typical ST.51 set up on it and would do the same today as follows: ST.51 engine, Glowdevil R/C long plug, APC 12.25/3.75 prop, 6 oz. plastic tank set up for uniflow on pressure, Scott Dinger muffler, and SIG 10% Champion or similar fuel. The ST.51 may run well for you on suction, but have things set up to try pressure just as a tuning and trimming tool.
Good luck and have fun with it!
Type at you later,
Dan McEntee