Ive done it twice.
Its takes a lot of work and not just for the motor, the rest (around the gearbox) takes a lot of time also. You'll need a dremel with a flex attachment and a bunch of sanding drums. A long, large diameter (1 inchish) drill bit helps to core the neck and the rear of the stock for the battery. The fit is trial and error as stated.
You are best off to try and get a drill stock avail at
Freds M14 Stocks. Unfortunately he doesnt ship to Canada, or didnt last I checked. The drill stock is thicker around the neck. When you core out the neck you have "c" hair of fudge factor before you poke a hole.
Its not that hard, just a lot of stop and go.
Good luck.