Thank you for your replies! They are very informative. :) I wish though to clarify a few more things.
I forgot to mention that I need to run it also at the lowest RPM possible. How can I demand, say, 1.5kW at a speed of less than 2000 without the engine shutting down?
I have a task that only needs around half of my single-cylinder diesel engine's rated capacity.
The question is, what is the simplest way to run my engine at partial load without sacrificing efficiency? The obvious ones are reducing displacement through a sleeve, or possibly an EFI conversion...