Increase RPM/Power of Air-Cooled Diesel

Hi,
It's about a real dinosaur of diesel technology, the Toyota 2H engine.
This beautiful 6-cylinder inline engine (102 mm bore) with a displacement of 4.0 liters has a rated power output of 76 kW at 3500 rpm. The engine speed is approximately 4200 rpm.
The engine has a lower camshaft with a valve mechanism via rocker arms and levers. Bosch/Kikki Single-injection pump with drive via bevel gear.
Therefore, the engine is exactly as I always wanted it to be
Damage to the engine over its operating life is rare. Typically, after a mileage of 500 to 800 tkm, the vehicle's body (Toyota HJ 60) is prepared so that the engine can be sold to a boat builder as a marine engine. In fishing boats, the engine gets a second life
This is the engine where the shareholders send the designer into the desert because it is too robust.
What do I want to know now, after this long introduction:
Can the fuel injection pump be "tuned" so that the engine runs freely up to around 4000 rpm? Are there a few extra horsepower to be found above 3500 rpm? 76 kW at a 4.0L displacement is very low, even for a naturally aspirated diesel. In the standard configuration, the engine can only be driven above 3500 rpm with brute force, it's running against a rubber wall. I don't want to make it a high-revving engine, but the extra 500 RPM in the rev range would be very beneficial to me.
Does anyone have any ideas about what we can do?
By the way, building a turbo is out of the question; I don't want any of that junk on this old engine
There was also another engine in the Toyota lineup, which was used in the successor model, the HJ61. The same engine block was used (same bore/stroke), but it was equipped with direct injection with a rotary injection pump and a turbocharger. This engine achieves its rated power at 4000 rpm, so there is nothing mechanically against that.

Thank you and best regards.
Jens