likely it would've looked like a standard KingAir 90 with lengthened nacelles (I'm just guessing the -135s are longer than the PT6As usually used). In other words, it might look similar to the Walter 601 King Air conversion (yeah, it might seem weird to retrofit a plane with engines nearly identical in performance to the ones it came with from the factory, but a complete 601 unit costs about as much as it takes just to overhaul a PT6, or so I've been told):
As you can see, it's not much different from a standard model.