Mine has a ST .51 with a MACS muffler and a Tru Turn Spiner and it was still tailheavy. I got a 6volt NiMh battery, It is a little heavier and pushed it as far forward as I could. I had it in the space under the fuel tank and that was a little too far forward, I ended up with it mounted vertical against the bulkhead between the fueltank and forward of the aluminum wing tube. it fits snug and velcroed it in and it balanced perfect. The 6 volt battery speeds up the servos and gives a little more torque and that makes the extra weight useful instead of dead weight like lead. About the OS LA series, I have heard stories that they are a weak engine, reliable but gutless, it is a bushing engine and it seems to me there is a lot of internal drag, I have always had, and will try to stick with ball bearing engines, a little heavier but they have more power, and a heavier engine will help with the CG problem