Leave the barrel broken and dont locate cocking arm yet. If you have a repeater, there is a small "O" ring on the cylinder that seals that side of the loading plate---check its there and in good nick. I made one up out of leather and it seems to be working ok. Replace the repeaters brackets and the loading plate on the top of the cylinder. Once the front bracket and loading plate are in place, then you can close the barrel and locate the cocking arm. Replace action in frame, replace two front screws and the large rear one (and its lock screw if it has one--if the hole is there but no screw, I think its 3 0r 4mm---I got one from local hardware shop and cut it to length). Replace plastic and sight.
Id hate to have to order spares for these because there are so many little variations. If you have the repeater and the magazine and rear bracket are missing, you still need the front one because of the loading plate. The barrel is different to the normal rifle. AS I found out, DONT BUY A BOX OF BITS-- the mk1 and 2 frames and cylinders are diffreent as is the one with the Gamo type slot in trigger. Keep the power down and you will have a fun, accurate plinker to put a smile on your face for years to come.