[Accelo]

No spanner needed.
Once you remove the screw and retainer you can put the cable back on to protect the threads and pull on the cable nut, with a pliers if you have to. It will come right out.
The three numbers (40-42-42) are the corresponding gears that go with the particular housing. So you should have one of the three gears in there. Other ratios require different housings.
There are two seals to keep this part from leaking. An "O" ring on the OD of the aluminum housing and a small seal that goes in the center hole and keeps the transmission oil from leaking past the gear.
90% of the time the plastic gear is worn and will require replacement if you want it leak free. Only hand tighten the cable. Tight will not keep if from leaking!
Yes most shops will carry the gear. Get a new one if you can and the two seals while your at it. The gears are also available on eBay.