If one wants to go to the trouble of re-drilling the engine mounts, the lightest one I found (GP, Hayes, etc.) was the Goldberg type that has two seperate rails. This mount is not only lighter but allows you to push the engine back right next to the firewall and shift more of the weight back. The two strokes that seem to be the best power to weight for this plane are the OS 55 AX and the K&B 61 twister. Both are lightweight and more powerful than the typical 46 and similar in size. Will a twister swing a 13x4 well?