I'm not very experienced in the certificated world...but I can't see where anything prior to 1993 is going to matter much today, documented or not. Especially after you installed a brand new engine.
There are many reasons, certificated or not, for wanting complete logs.
One is documentation of airframe time. Does the plane have 2,000 hours, or 20,000 hours? Given a choice between two identical planes, same price, but one with 2,000 hours and one with 20,000 hours, which would you pick? Of course the one with 2,000 hours. Which begs the next question - how much less are you willing to pay before you'll pick the one with 20,000 hours?
Another question logbooks can answer is if the aircraft has ever been a trainer. If it's privately-owned on leaseback to a flight school there'd be no paperwork trail of its use as a trainer. But a series of "100-hour" inspections in the logs would be a big giveaway.
This came up with Piper Cherokees and the main spar AD. The inspection is pretty onerous so a formula was created to determine if an aircraft qualifies. Landing stresses on the main spar are higher on a low-wing aircraft, but landings are not recorded in U.S. logbooks. So the number of 100-hour inspections was used as a proxy. The theory is that if a plane is undergoing 100-hour inspections it's being used as a trainer and therefore has a large(r) number of landing cycles per hour than a privately-flown plane. So the formula took into account total airframe time coupled with how many of those hours were subject to training (100-hour inspections). If either (or both) numbers were too high then you had to pay for an expensive inspection (and hope you passed). If logbooks were missing then the only safe avenue is to assume the worst and do the inspection.
Now, Cessnas are not subject to such an AD. But what if...? A future owner would want/need complete logs to be able to answer such questions.
Then there's the "FUD" factor (Fear-Uncertainty-Doubt).
Why are the logs missing? Is it a simple mistake, such as lost in a fire or flood? Or because the aircraft was exported to Canada where U.S. logs are often tossed in favor of new, arguably more comprehensive Canadian logs? Or was the person trying to hide something? If so, what? It's the last question that lingers, unfortunately.
My current ride was advertised by the broker as a "one-owner airplane!" Sounds great, but I told the broker, "don't try to snow me - I know that 'one-owner' was a flight school!" I bought the airplane knowing full well it had 10,000 hours on the airframe and paid accordingly. I also had documentation to prove it. Some Cardinals (not RGs) are used in pipeline patrol. They have 20 or even 30,000 hours (or more) on their airframes. Is that a problem? Don't really know. But it certainly affects their perceived value.
