Brodak offers a conversion kit for the Legacy, check their web site. Do you have the Legacy kit, or do you have the ARF/ARC version? I converted a Legacy ARF to electric and the process was fairly simple. Removed a portion of the motor bearers (some kind of hardwood) with a rotary grinder, then installed a ply motor mount (1/8 inch hardwood ply), with bracing gussets to support the mount. I can post a photo if it helps.
If you have the kit, you can just delete the maple motor bearers and design your own motor mount. I have an unbuilt Legacy kit (I really like the Legacy) and can check the kit-supplied doubler thickness. Honestly, the electric power is more than adequate to pull the model without being overly concerned about weight. My ARF Legacy is 64 oz (??grams) with battery, ready to fly, and has no problem with power. You could substitute 1/32 (??mm) ply for doublers since there is little vibration with electric power, but again, I don't see any real need or reason to go to the trouble, time and expense of doing so.