In addition to the stackpipe O rings you will require 2 PTFE back-up washers. These are flat nylon-type spiral washers that resemble a double coil spring washer. The O rings should be fitted first and then the PTFE rings wrapped around inside the O rings so that they are facing each other along the length of the stack pipe....NOT with the O rings between them.
OD 13mm 1/2"
ID 9mm 3/8"
Section 1.5mm 1/16"
O ring part number 195 561M1
PTFE washer 195 874M1
Pin 897 504M1
Roller 897 599M1
You will require a third O ring to go between the cap or valve on top of the cover.
From your photo the tractor has had both types of retainer in the past. The popper (upper right) is inserted into the backplate of the pump and then into the pump lever swivel bracket. It is this type that is easiest to dislodge if you do not have absolute control over the hydraulic cover when refitting it. The clip in the middle is used with the grooved rivet on earlier tractors before the change to the popper.
Fitting of the pump lever top roller can be awkward. If you should lose it you will have some fishing to do. I have a pair of bent long-nose pliers which I have ground a groove in near the point ends to hold the roller. With a cable tie slid over the pliers handles that locks the roller in the pliers until I am sure that the roller ends have located and I can slide the cable tie off.