Fox no longer manufactures the 15X. The 15X weighs a little under 4 oz as I remember, slightly LESS than an Enya .09! The Enya 20 is probably too much for the Akromaster. It will still fly but will likely be too heavy to stunt well with the standard wing. Remember that with a larger engine you will need more fuel and a larger tank making the plane heavier still. I would lengthen the wing as previously stated in this thread if I was trying to use a .20 size motor.

The auction sites often have Fox slant plug 15 engines for less than $25.00 and steel finned .15 for even less.

My Akromaster with a Fox steel finned 15 flies 3.5 second lap times on 45 ft lines. Pretty fast! I was flying 4 second lap times with an Enya .09.

With a .20 in the nose you will probably need 55 ft lines.