You might consider the engine fit from a Dash 8 as I remember those engines are about the right power rating to replace R2800s, but those engines came along rather later than most periods you'd want the B-26T for. Given that the longer nacelles would likely interfere with the pilot's vision, I think Jeff's idea of using R3350s makes a lot more sense (from my point of view, it makes lots of sense from the aircraft weight and balance situation, too).