Ty, bent crankshafts, if they happen in service, nearly always are bent on what B&S call the Power Take-Off (and I call the output shaft). So, if you can lay the main bearings in a pair of V-blocks, and put a flat metal object just under the far end of the PTO so it just barely clears it, you can measure the runout easily. Just rotate the crankshaft on the V blocks in small increments, measuring the clearance between the tip of the PTO and the flat block underneath it with ordinary feeler gauges. If that clearance varies by more than, say, 0.010" as you rotate the crankshaft a full revolution, you have a bent crankshaft. (Note that a total variation of 0.010" is what is known as a "Total Indicator Reading" or TIR measurement, implying that the center of the crankshaft is bent 0.005".)
The hardest part of all that is getting hold of a pair of V blocks. They are not easy to improvise, because the main bearings have to lay in them perfectly, not moving at all when you rotate it.
Those of us who are into machineshop stuff would use a magnetic-base stand with a dial indicator to measure the runout, but that isn't necessary, it's just a bit quicker than the feeler gauges.
