Posting here on behalf of my brother with a dead LT155..
Story as I've understood it
1) was recently serviced and running fine
2) his son was mowing with it, stopped and turned it off to go do something, everything normal to this point
3) Went back out to start it, and reports a "loud rattle" but no start, engine cranking OK
His diagnostic steps so far
1) checked spark at spark plug by grounding to engine.. There is spark
2) Tried starting with starting fluid.. nothing..
3) replaced some crank key that apparently affects timing?
Now, he says with thumb over spark plug hole there's no obvious air movement, which might indicate low compression, or that there's no piston moving, but it seems like something so dramatic as a broken con-rod, crank etc, would make a terrible racket when turning over..
Doesn't seem to be a fuel problem since he's tried starting fluid.
His next step is to remove the head and inspect for damaged valves/piston etc... Any other diagnosis he can try before that?