I can't say for sure, but the air filter on my EL300 A-R Kubota diesel, 4hp, liquid cooled 2000rpm, was marked 4910hrs. The 'Bota came from a flashing arrow sign, with an alternator bolted to it. Still starts in the summer without glow. I have no idea if it was ever rebuilt, but it doesn't look to have ever been apart. If it was, someone painted it afterwards. I figure that's pretty reasonable for a 300cc single, and it's far from dead yet.
I also have a MDJA Onan diesel, one cyl liquid, hours unknown, a bit tired, however it came from a small reefer, and has a centrifugal clutch on it to allow idling, so you KNOW it ran a LOT of hours. Holds 5-6 liters of oil I think.
My own truck has a 4 cylinder Isuzu diesel, with an estimated >10,000 hrs on it (550,000kms), and it is also getting tired, however it still starts instantly warm, and gives me 45mpg in the summer.
Personally, I believe it is very possible to get good value from a high speed diesel.
That said, I still want a Lister 6/1 ! WAY more cool than a high-speed diesel.