Concerning the ST .45, it is a great engine, but heavy for a plane the size of the Kobra. The Webra .35 has about the same hp rating (for whatever that's worth) as the ST, but weighs in at about 6 oz lighter. And 6 oz. on a 3.5 lb plane is a LOT of weight. The ST .45 will work fine, but you may find yourslef adding tail weight. Extra weight will affect vertical performance, increase stall speed, make landings more difficult, scrub off top end speed and increase the tendency to tip stall leading to the spiralling snap roll of death. You'll have a rocket, but overall flight performance will suffer with every added ounce. As with ANY plane, it's all about weight. If you want a .40 or .46 engine go with the LA engines to save weight.