They got it right first time.
First time? Not quite...... The PC6 Porter with PT6 engine was the last of the line.
Originally powered by a Lycoming GSO-480, then Lycoming IGO-540-A1A (both dogs!), then an O-540, then Turboméca Astazou of various models, then Pratt & Whitney Canada PT6A-6A and -20, finally the -27. I think some Porters have been retrofitted with the -34 engine under an STC.
The Fairchild Industries license built Porters were fitted with Garrett TPE331-25D, then Garrett TPE 331-1-100 and -101F (Air America) and a 500 HP Lycoming turbocharged piston engine.
Between May 1959 and this year, 562 aircraft were built (still in current production.)
They were always a "special purpose" aircraft, not commercial. At least four operated in PNG, including one piston Porter. One problem I seem to recall was the max gross weight in Australia: 4,800 pounds (?) in Australia; 6,173 pounds max gross weight in the rest of the World and up to 7,500 pounds military configuration. (Empty weight ~ 2,800 pounds.)
The PC-8 Twin Porter was a twin-engined version flown in 1967, but not subsequently developed.
At one stage Pilatus also owned Britten Norman, building Islanders.