Hi Paul, How did you go getting the shaft straight and/or finding a replacement? I just got a 6HP B&S Horizontal shaft Model 117432 Type 0551E1 that has a stepped crank. Just wondering if it is something like the one you are after. The motor turns over okay but has too many other things wrong to make it a restore possibility so if the crank is interchangeable I could strip it out for you.
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The stepped area is approx 60mm long and 15mm diameter.
Hopefully someone will be able to advise if it will fit if the step is like yours.