>>>Not the bolt, checked that. When I rotate either wheel, the opposite wheel spins with it in the same direction.
That would happen with or without the bolt broken as it’s a straight unbroken shaft.
Unless you just bought this thing or working on it for the first time what happened the last time out? If something went it probably would have been noticeable the last time out like a snap or grinding of trans internals. I don’t think the internal gears are keyed.
It’s looking like a broken trans but some head checks would be:
Turn the wheels by hand in neutral with the clutch disengaged. They should be fairly free. Note the position of the transmission drive end to axle.
Turn the wheels by hand in gear with the clutch disengaged. I would think drag would increase slightly.
Turn the wheels by hand in gear with the clutch engaged. You probably can’t turn them without great effort.
Pull the start rope with it in gear and the clutch engaged. The machine should move slightly. Put a mark on the belt and transmission pulley to note movement. The way your machine is acting I would think you can pull the rope with only a slight increase in effort from usual as the trans internals are slipping for some reason. The belt and transmission marks should have moved.
