I'd have to go glow with this one.....perhaps a vintage OS 25 VFR with a tuned pipe back forward inside the fuselage, the K&B 3.5cc with the "barrel" type muffler would do, and last but certainly not least the OS Wankel with a straight pipe out past the prop!

Say Opjose, what size is the stock fuel tank, and with little or no modifications what larger size tank do you think would fit? The Wankel is THIRSTY!

Rube