First, welcome to the forum Brian. I agree, your Scotty looks to have been either little used or gently used, and should be an enjoyable project leading to a good result. By coming to Outdoorking you have come to the right place to follow your mower interest - there are many enthusiasts here, including some Scott Bonnar afficionados.

As a first step on your Diplomat, try this thread:
https://www.outdoorking-forum.com.au/forum/u...Words=diplomat&Search=true#Post17291
A search on this site for "diplomat" will turn up some other information as well. As you can see, the Diplomat was a belt-drive Scott Bonnar; they also made chain-drive models (with numbers such as 33 and 45 instead of model names), which turn up here more often than the belt drive ones.

You probably will not have much difficulty getting your 3 hp Briggs engine working properly, and of course people on this site will be happy to help. If there is a lot of water in the engine's sump though, you need to remove that properly. If there is plenty of oil as well as water, there may be enough oil film on the bearings and camshaft to have prevented corrosion damage, but you can't tell yet. Of course there will probably be water in the carburetor, if anyone has tried to start it with water in the fuel tank.

The slow retraction of the cord starter may be due to water/corrosion. You will need to remove it and service it. You can find information on its little mysteries with a search of past posts on this site - essentially it may have rust, and it requires careful cleaning and a rather special lubrication process involving one drop of oil in just the right place (the spindle bearing) and no oil whatever elsewhere in the starter. If you work on the engine one step at a time and keep us posted, we can probably help make this a fairly orderly process. The old 3 hp Briggs is a nice engine, and if it isn't damaged you will probably be happy with it.