I would never put anything less than a 50 on an X, it is the motor the boat was designed for. You will not gain any sailing advantage with a smaller motor and you give up all the powerboat advantages designed into the boat.
These days it would be a 4 stroke 50, the brand really doesn't matter much, pick the one with the best support in your area. This is the best combination for fuel economy, ease of maintenance and long term value of the boat. None of the available 50 hp motors will impact the ease of access to the cockpit. The X has plenty of space back there. It narrows the field some, but you might want to only look at 4 stroke 50hp motors that swing a 14" prop. These make the boat handle better at low speeds.
Now if it were me repowerering my boat, after spending 7 years with a Tohatsu 50hp 2 stroke, I would not put anything smaller than a 70hp motor on my X. I would choose a 4 stroke over any of today's 2 strokes. While the Tohatsu and Evinrude 2 strokes are far better than my old one, I still would just rather pass on the whole separate need for oil thing and I don't see the small weight advantage as a big deal.
I'm impressed with the performance of the new Mercury 75hp reported here, and I also think the new Honda 75 looks very promising on paper. Both are 1500cc motors which look to give great performance. The extra weight of these motors is easily offset by making smart choices on the placement of water and batteries (and even fuel). The Honda is very light to start with. I don't think I would spend the extra money for the 90hp version of the Honda which is the same physical motor. I want a bigger block motor for quieter operation in the mid RPMs, not top end max horsepower. I think I would get all the performance I need in the 75.
http://www.honda-marine.com/new_model.html
With a 50hp the X is just short of the needed thrust to truely get out of it's own hole, particularly with a multiday cruising load.
For the first time I experienced what is possible on a X this last week. I met Dano from this board up here in Seattle on Tuesday to help him sort out the used X he had just bought on Monday. Once we had the mess of all the loose parts installed, the mast and sails up, and the rest of the equipment gone through, we put the boat in the water. It has a older 90hp Mercury two stroke (it also has a 50 gallon fuel tank up under the vee berth). Once we finally got fuel from the tank primed to the motor (It's a long hose run to pump with the bulb) the motor started easily and ran well.
We left the dock at the Mercer Island boat launch on Lake Washington and headed north under the East Channel bridge cruising slowly until we were up by Luther Burbank Park. There we pulled the fins up and opened up the the 90 to see what she could do. For the first time in an X boat I could actually feel the bow clearly drop and the boat come up on plane. Unfortunately the GPS that came with the boat didn't have batteries in it so I can't say exactly what our speed was, but comparing to my boat's look and feel I would guess we were doing between 18 and 20 knots. The best my 50hp will do is 16 knots.
It is just this little bit more speed that is required to truely get the an X up on plane. I think it will get there with any of the 70+hp motors available today. I would not re-power my boat with anything smaller.