Matt,

Do you have any idea how your balance is going to come out? I ask because it can have an influence on the engine choice. If the nose of the Altair is not long, and your airframe may tend toward being tail heavy, I would consider the Zenoah 445. Not the lightest in that size class, but a good match. Magneto ignition adds to the weight, but takes the headache of ignition batteries out of the equation. If you are going to add nose weight anyway, no penalty. OTOH, if your finished plane is tending toward being nose heavy, there are a number of good engines, 3W amoung them, that will have plenty of power and minimum wieght for a two banger.

Good luck!

Bedford