Re: Gravely question
Hard to tell. Since the tractor was repowered, I have to wonder if the problem might be with the gear on the engine output shaft. It's a floating gear in that it is not c, or e clipped on the shaft. It slides on the shaft, but normally meshes perfectly with the transmission gear when the engine is mounted to the transmission. I wonder if either the gears didn't mesh quite right, or the output shaft gear key could have fallen out somehow.
It could also be something real simple like the high/low selector being out of adjustment, or not fully engaged. Or something more involved inside of the transmission. That wouldn't likely be the end of the world either. You would just need to take your time as you work your way around the transmission.