I dare say that the engine should outlast the rest of the tractor.
BUT there are some qualifiers....
Any engine is only as good as it's maintenance. Good (or bad) maintenance can double (or halve) expected life.
Lack of use also affects lifespan. Frequent start/stops, not letting engine come up to temperature, etc shorten life span as well. Extended periods of non-use allow oil to drain, rust to form, and water to condense in oil.
Type of use - diesels actually do better under load. Something most of our tractors dont really see that much in casual use.
Fuel quality
But the mitsu's are good engines - been used in lots of industrial applications for years.
8-10 thousand hours is a reasonable expectancy for a maintained engine. The smaller ones may not get quite that much, but you still have a long way to go. The secondary systems (hydraulics, alternator/electrical, clutch/transmission will usually be the death of the machine before the engine goes bad.
For example, the small diesels that run refer units on tractor trailers have a life of 15,000 hours or more. Different type of application, and they run nonstop for most of their life in a "protected environment" with little variation, but shows you what is possible.