I have Briggs 5S and 6S they are similar to the cast iron engine you have, but not exactly the same, Maybe it is a Briggs, but anyway I like the DIY work your up to and nice seeing it coming along!
I see you already tapped the crankshaft for the roller. I did a 10-24 thread tap on a newer Briggs 80202-0430 70-80's era engine that was an aluminum design. I just was careful and drilled by hand. It worked for my need to just add extra security besides the keyway/set screw, by using with a flat/lock washers and bolt to use on my centrifugal belt drive clutch.