I don't recall any ST's having DeSaxe although it's possible but the Fox 29/35 are certainly DeSaxe. All of the original Mercos (29/35/49 and 61) were DeSaxe but I seem to recall (at least with the 35) they were offset to the opposite side to the Fox. Also an SC 108 that I have is DeSaxe but the 120 isn't. There's definitely a difference in port timings either side of BDC but only a matter of a degree or so.

Just to add a bit to the dwell time at top and bottom dead centre, con rod length/stroke ratio plays a big part in this. The higher the ratio the better and around 1.8 is desireable.