The Magna is a V4. It's not like a V-twin in lots of ways. It's never going to sound like a V-twin. You could change the mufflers on it but if it was louder you still wouldn't like the sound it made.
It's a wonderful engine though. Very refined and smooth, wide power band, relaxed cruising. A friend of mine has an old Magna and though I'm not a cruiser person myself I think the engine is the best thing about it. I bought an ST1100 just because I loved the V4 so much.