Replace barrel and barrel pivot bolt and the lock screw. You can use or make up a bit of shim so the cut outs in the bolt head line up with the lock screw. Replace cocking arm and its pivot and lock screw. Put action back in stock and tighten up stock screws. Replace sights.
I find these a very well made,robust gun and an ideal starter to strip. The only real problem on the British ones being lining up the trigger when replacing the pivot pin. The pistons are much better than the 22,s and I reckon if you have a decent piston and breech washer and keep an eye on the pivot screws,will probably not need doing again until you need a new mainspring. I think meteor springs will fit the 27 and are a touch "softer" although there is not much in it. If the trigger goes light, you can file or stone the trigger face square (and /or ) the piston rod face. You can also play with different bits of trigger spring to get a feel you like. Manchester Airguns used to sell lengths of spring for a few quid and this was ideal for playing with trigger and barrel plunger springs.