It is possible to fly just about any aircraft as a single pilot, given enough time to prepare.
Probably, though I cannot see why.
FAR 125.263 says that
(c) On each flight requiring a flight engineer, at least one flight crewmember, other than the flight engineer, must be qualified to provide emergency performance of the flight engineer's functions for the safe completion of the flight if the flight engineer becomes ill or is otherwise incapacitated. A pilot need not hold a flight engineer's certificate to perform the flight engineer's functions in such a situation.
Somewhere else in FARs, cannot find where, is the requirement that no transport plane may become unsafe by incapacitation of any one flight crew member. Two pilot planes meet this by ensuring that either of the pilots could fly the plane alone.
But what about three required crew planes? There s no requirement that any plane should be flyable with two crew incapacitated. Two pilot planes routinely crash with both pilots incapacitated (see Helios).
So, there is no requirement that it should be possible to fly a three flight crew plane with two crew incapacitated and one pilot left. Can they in practice be flown so?
Also, when one of three is incapacitated, is it safer to fly with both pilot positions occupied and FE post empty, or with FE post occupied and one pilot seat empty?