As well as pure prototypes I would love to see some of the re-engined aircraft that abounded in the 60s and 70s such as my personal favourite the
Saunders ST-27 a completely re-engineered De Havilland Heron http://en.wikipedia.org/wiki/Saunders_ST-27
Saunders ST-27: The fuselage was lengthened by 8 ft 6 in (2.59 m), to accommodate up to 23 passengers. It was powered by two 750 shp (559-kW) Pratt & Whitney Canada PT6A-34 turboprop engines. Twelve Herons were modified by the Saunders Aircraft Corporation of Gimli, Manitoba, Canada.
and the re-engine kits for the Heron offered by Riley aircraft:
Riley Turbo Skyliner: Re-engined aircraft. A number of Herons were fitted with 290 hp (216 kW) Lycoming IO-540 flat-six piston engines. The modifications were carried out by the Riley Turbostream Corporation of the USA.
and the Japanese Heron
Tawron: Conversion by Shin Meiwa of Japan for Toa Airways with 260 hp (194 kW) Continental IO-470s replacing the original engines
The Saunders would have to be a separate type - but surely the Heron could be offered from say 1975 onwards with the newer engine options I doubt it is possible to opt to re-engine aircraft from the used market - say as part of a D check on acquisition?
This of course opens the possibility of Dart engined super DC3s and extending the life of Convairshttp://en.wikipedia.org/wiki/Rolls-Royce_Dart
Some Douglas DC-3 transport aircraft have been upgraded to use Darts. DC-3's in BEA service with this update were called Pionairs. Another conversion is the Conroy Turbo Three.
Convair 600 and 640 converted Convair 240, 340, and 440 aircraft there were 240 of these conversions so they deserve a spot in the game - anyone say Trans-Texas Airlines?http://www.aviastar.org/air/usa/convair_600.php