Check the transaxle! A simple way to check is to feel for excessive axial play in the propeller shaft right before it enters the transaxle. There should really be no play/slop at all, and if so, there either already are problems, or there will soon be some.
The rest of the tractor is a tank for sure, with grease fittings all over the place, and very heavy gauge steel throughout. It would have been nice if there was one on the steering wheel shaft, but that could be added easily.
The main weakness is the transaxle. The input shaft is not properly supported with only one small radial ball bearing on the outside, and a very small roller/needle bearing behind the front cover of the transaxle. The distance between these two bearings is very short, only like 2 inches, yielding very poor axial load capability. Couple that with a pretty heavy and long propeller shaft, at less than optimal angles, and you have a nice sinusoidal axial torque beating the weak bearings. There is more, but that should be enough for now!