While it would have been better to change the oil at 50 hours, or after a year or so, if the oil hasn't been diluted with water, and the filter is doing its job, it should be fine. If the tractor was 18 months old, and had 70 hours, we wouldn't be having this discussion, so sitting for a couple years without being used might not hurt anything. Heck, change the oil, and change it again after another 50 hours, if it makes you feel better. If it runs fine, it probably isn't damaged.