Tower is selling the OS 91 SX-H for $215, which should be a good choice for UCD. The airplane version with its remote needle valve is just not worth it. The stock muffler is heavy and power-robbing , so you'll want a Jett or Ultra-thrust muffler anyway. This helicopter engine has 3 needles valves, and should have better midrange and transition than the 91 FX. Y-NOT is using the 50 SX-H on his Flip with great luck. So this SX-H should work well for the UCD.