I have an old 68 3hp, and I've done maintenance on it over the years.  old family motor, and it runs better that it ever has last few years.  I saw an ad for a 71 6hp, were those build well too?  I would assume so, but different years and hp motors can have inherent weaknesses or problems.  The recoil on the thing seems odd.  The one of the 3hp works good, but it is a different mechanism, and is kind of a pain.  The 6hp recoil, never seen anything look like that, is the recoil mechanism a problem on these motors?  or some other common weak spot to know about?