The larger MDS engines are SUPPOSED to be pretty good. The 46 (and smaller ) and many of the 61 are best used as door stops or fishing weights. Do a search here on MDS and draw your own conclusions on why ther are reffered to as: Metal Door Stops, Mostly Don't Start and Many Dead Sticks (those are the "nicer" comments about the smaller MDS (up to and including 46 and some 61 ).

On OS -

La is a bushed bearing engine, about 1/2 the power of an FX/AX.

FX/AX are ball bearing engines. Quite powerful (especially if the baffle in the muffler is removed ) and rear needle valve with a 2 needle carb. The AX is a direct replacement for the FX. The differences - AX has a plastic backplate and a 4 bolt head rather than the 6 bolt head. Both are very user friendly and typically run great right out of the box with minimal break in required.

You may also want to consider a Thunder Tiger Pro 46 (front needle valve ) and the Evolution 46NT. Both these engines are equal to an OS in power, durability and user friendliness but are noticeably less expensive in price. For a 4 stroke, spend the extra money and go with a Saito. My 3 Saitos have been trouble free, EXCELLENT running engines. YS are more powerful, but more expensive and do require a lot more "fiddeling" with to get running properly and keep running properly.