Given the conditions at which you noticed the failure, I would suspect a head gasket first.
It seems like you have noticed you are loosing coolant over time, and the condition which it began to loose power was likely working the engine a fair amount -- running pretty hot. Have you checked your oil to verify there is no coolant mixing into it? If you are able to start it now, I would let it run for a bit while keeping an eye on your coolant temperature. If it overheats now you are likely dealing with a head gasket problem. You could also run with the radiator cap removed and look to see if you coolant system is bubbling. Both of those would be indicators of head gasket issues.
Second possibility would be that you are over fueling and causing a cylinder to smoke instead of burn completely. Now that you have the tractor running again, is it running smooth or does it run rough? If it is running rough, you may have an individual cylinder problem that we can pinpoint by cutting the fuel to a single injector.
Those are some ideas on where to start. I am linking my troubleshooting thread as well, which has some links to videos and pictures of isolating a rough cylinder, smoking and fuel issues.
https://www.tractorbynet.com/forums/chinese-tractors/418264-farm-pro-2425-engine-knock.html
Last but not least, if you don't find an obvious sign in the simple diagnostics above - harbor freight sells a decent diesel compression test kit that can thread directly into one of the glow plug ports.
Hope that gets you started. I beat my head against a wall for a while until I found my initial problem was rather simple. These engines are relatively simple but I had no diesel experience before I got started into mine. Good luck!
Eric
