I haven't worked on a 125 (or any tractor in that series) but I had a 147 which I think would be similar.
remove the four bolts that hold the engine to the frame, two above the axle (need to jack up the front and let the axle pivot down to reach the bolts) and the other pair about 6 inches back. That allows you to jog the motor forward an inch or so to release that end of the driveshaft.
Remove the rag joint bolts, the shaft should come out.
Either style would work, so it doesn't matter. My 147 had a welded yoke. Might as well put a new flex disk in when you put it back together unless the one you have is in particularly nice shape.
Ditto on the advice to check out the manual.
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