Hello Giannis,
When i've had to model forklifts, i have used one of two options:
Diesel burned in agricultural machinery - represents the burning of 1 L of diesel in a tractor
Diesel burned in building machinery - represents a machine representative for the industry.
Be aware that the complete combustion of a L of diesel will emit roughly the same amount of CO2 every time. If you are equally interested in other impact categories, investigate the processes, and see if you can identify which process uses a filter that is more similar to your own scenario.
I hope this helps :)
