Small Engines (Lawn Mowers, etc.): Craftsman B&S 6.75 Self-propelled, drilling the hole, locknut

Question
Model # 917.378922 B&S 6.75 hp, front wheel drive. I just bought this Mower used.  Noticed it has more than normal vibration (at idle) but engine & drive runs good.  When I was removing blade for sharpening I noticed that one of the alignment pins in the blade adapter/pully (#187037) was missing.  1st thought it is was a (threaded)bolt missing but from what I can see there are no threads in the hole.  Without the other pin the blade does not align and balance properly and I'm sure that is casing the vibrations.  What are my options here?  Can I get a replacement pin? Can I drill it out and install a bolt or do I have to (remove) replace the adapter?  I don't have a puller. My friend suggested just drilling the hole through and installing a 3/8" bolt (& locknut) Would  this upset the blade balance?  Is there a simple fix?  Thanks Steven :-)

Answer
Normally we would replace the blade adapter.  Either case, drilling or replacing, I would remove the blade adapter and check the internal key to ensure it is not damaged.  With the spark plug removed, have someone pull the sting while you watch the blade.  Does the blade appear to wobble?

Let me know.
Eric