If oil is getting up into the speedo then it suggests that the driven gear part No67-3175 is worn or the gear bush and should be replaced, fitting an O ring on that spindle will need to have groove machined into the spindle this will mean having access to a lathe as to size that would be determined by the depth of the groove as the inside size of the O ring and the outside size by the diameter of the gear bush. You would also have to be careful not machine too great a depth of groove in the spindle as you would weaken it. I would take the easier option and replace the worn parts.