I recently acquired a John Deere 425 with about 1300 hours and a sixty inch deck. It was in pretty rough condition and hasn't been used regularly for two years. The rear end leaks which I'm pretty certain is the diff linkage seal. I have pushed it back in and it reduced the leak to almost nothing. I'm going to replace at some point. My main concern is when I got it out seemed very underpowered. I couldn't now up step hills unless I was at a crawl. So through some research and troubleshooting figured out it was down a cylinder. I ended up pulling the valve covers and finding the intake valve push rod was bent and off the rocker and the exhaust valve push rod was straight but also off the rocker. I replaced both and now it runs quite well. Very strong even with the big sixty deck. I should also add that the exhaust valve was stuck so I sprayed with penetrating oil and worked it up and down until it moved smoothly. My concern is why did it bend the push rod in the first place and if I need to do more to ensure it isn't further damaged. It has run about one hour since replacing push rods with no issues.
I know nothing about this tractors history so not sure if cam gear has been replaced. I sorry assume it has but only because the hours are pretty high if it hasn't and clearly it hasn't been well taken care of.
Any thoughts on it I should just keep driving it and not worry unless it happens again or if I should rebuild engine. Or something else entirely.