Facts: If it is a 445700 series engine, it is not an "opposed twin".
If it is an opposed twin, it is not a 445700 series engine.

You had better find the Model and Type numbers of the engine before proceeding any further.

Because.....if it is in fact a 445700 series engine, your most likely problem from the description you have given about the cranking (stalling of the starter) would be the need for a proper valve adjustment.

The lack of "firing" may be an unrelated secondary problem but we must know the correct engine identification before any helpful advice can be offered.
You probably need to include the brand and full model number of the tractor this engine is in considering there might be a safety interlock problem resukting in no spark.

Sorry, I'm an idiot.

Because the fuel line seemed to split at what I assume is a fuel pump, I thought it was a twin. Which was odd because I only recall changing one plug!

It's a single. Can't read the label on the side - it's faded & weathered so only the bar-code can be partly seen. It is the "Craftsman" labeled 18.5HP OHV Intek plus.

I can understand valve adjustment but why would it have run fine for years then suddenly not start after having sat over the winter?
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IF you have the Craftsman 917.xxxxxx number, we can probably identify exactly which engine you have.

The reason you need a valve adjust is that it affects the compression release.

Make sure to use the PROPER procedure.