Your engine was made on 18 August, 1984. If it is the original one fitted to your mower, the mower was probably made about six months or so later. The engine is 8 cubic inches (130 cc) displacement, of the first design generation, has a horizontal crankshaft, a Pulsa-Jet carburetor, plain main bearings, and a recoil starter. That engine was made for a long time and has a good reputation. It was the most desirable of the engines fitted as original equipment to the SB45.
Your reel looks as if it has never been reground, and the dings in the blade do not look very serious. However you need to get an expert's opinion on that. We traditionally recommend that people contact their local bowls club and ask to speak to the greenskeeper. As him/her where they get their own mower reels and bedknives sharpened. (Yes, both of yours will need sharpening, especially after the mishaps that have befallen that mower's reel. I hope you'll be a lot more careful to remove all stones and foreign objects from the lawn than the previous "tenant" of that mower was.)
To remove the drive shaft, hold the shaft in a Stillson or gas plier and unscrew the nut retaining the sprocket with a socket or ring spanner. The thread is a normal right-hand one. If the sprocket turns out to be rusted onto the shaft, you will need to soak it in Penetrene for perhaps a day then remove it with a small gear puller. If it isn't rusty it should come off fairly easily. It is on a key.
