Oil vapor could be getting on the points, you won't see it, but it will limit current, as oil is an effective insulator. Blast the points with brake cleaner, and see if spark improves.
Awhile back, I had used freeze off on a flywheel nut, After removing the [ good firing ] magneto, engine had bad knock, and after putting it on another engine, I had no spark.
I later changed points [ old ones were nice ]
and it started right up. When I closely examined the old points, I found a slight film of oil on the contacts. CRC Freeze Off penetrated so well the oil made it all the way to the inside of the breaker box, and fouled the points.