The mounting holes on a Fortrex do NOT...repeat NOT...match up with the holes from my Motorguide. My MG is 12 years old which may have something to do with it, but it's a Gatormount and the bolt pattern on the MK is narrower than the MG and the holes are spaced differently . At any rate, got the old motor off...tomorrow I'll get some silicone to plug the old holes and the right size drill bit (9/32) to mount the new one.

Make sure the holes are dry, or you'll be wasting your time with the silicone.  If you can get beneath the deck, put tape over the holes to stop the silicone from running out.

It might be easier to get a piece of dowel, slightly larger than the hole.  Sand a taper on one end, put some waterproof glue on the dowel, and tap it snugly into the hole.  Then cut flush with the deck.  Repeat three more times and you've got an absolutely watertight plug in each of the old holes.

Do put silicone in the new holes before installing the screws.  It will seal the drilled edges.

My profound apologies regarding my original posting in this thread.  The mounting holes in the Fortrex do indeed match a MG Gatormounts holes.  You have to remove the right side cover from the base mount on the MK and you'll find holes drilled there to accept the MG pattern.  I was running on 2 cylinders yesterday and shot my mouth off before making sure my brain was loaded.  Sorry guys......... :-[