When my fuel tank is less than half full my engine is beginning to run bad. The emptier the tank, the worse it gets. This way I can only drive for about 140 km (80 miles) This surely has something to do with fuel pressure, is there any way I can increase the pressure? May be a different fuel pump?

It aint pressure related directly
you got crap in that thar tankie my freind,clean it out.....or yer vents be blocked,clean them too

Is the fuel pump actually working ?

fuel pump is working, i feel a pulse when i hold the fuel line. Thanks for the tip about the dirt in the tank , i will clean it and also the carbs as soon as i come back from my holiday in the holy motherland Italy.

while you`re at it replace the fuel filter if it`s dirty and remove the fuel petcock from the tank. There`s a wire filter on the inner side of it you can check/clean. Do you have the original setup and carbs btw ?

G.

First of all I'd suggest you to check if the fuel pump is working properly. Release the fuel pipe close to the carb and turn the main switch on for 60 seconds. Do the same test with the tank full and almost empty, comparing the amount of gasoline in a proper bowl. You'll be able to see if it's true or not.
Another filter to clean is the one inside the petcock: remove it from the fuel tank and be careful not to damage its seal

Hot tip ! remove hard plastic sealing washer and substitute with an appropreately sized O ring. Seals better without having to excessively tighten nut ( this is real helpful if the threads or the sealing surface arn't great)