51pan cylinder crack

No replace it.

Replace it before it becomes a grenade

After a closer look still replace it. Though I now see it is a sleeve install in a cracked cylinder with holes drill at the ends of the cracks to stop propagation of those cracks.
If the other cylinder is also sleeved, I would replace it also.

I'd junk that cylinder or use it as a door stop. There are decent aftermarket cylinders out there that are fairly inexpensive. Whatever cylinder you use has to be bored to match the piston and honed for new rings, which is also not too costly.

That's a big crack and definitely needs a replacement. A friend of mine recently acquired this Panhead project and the previous owner mentioned the cylinder crack. We'll take a closer look soon after we finished installing the bumper and amp research board on the truck and clear the garage.

Braze it up.