question Ursus 385 power steering pump

Hi all, first time posting so go easy on me
I have been looking online for a new powersteering pump for a 79 Ursus C -385 and there seems to be 2 models.
1 is a 16 litre pump flow and the other is 21 litre pump flow. The tractor has a 16 litre in it but has always been stiff and more so when it heats up after a few minutes.
So wondering if anyone has fitted the 21 litre?

Putting a different pump into it may not improve the lightest of the steering, you could try that still be much the same also you can fit a more modern filter and new oil in the tank, you may also need oil in the distributor. Sometimes it improves if you fit a new coil in the distributor or recondition the block on the axle but the only way to get light smooth steering is to fit an orbital unit.