What is required to reverse the rotation? Say you have a standard BB, what is needed to reverse it? Does it have to be totally rebuilt, or just swap say cam, dist gear, etc?

Starter.

Cam has to be gear driven so it
still rotates l/h to drive oil pump
properly.

Pistons have to be rotated
180* to change pin offset ( the notch that usually
faces the front of the engine needs to face the back)

Crank seals can be rotation specific.

I'm thinking the circulating water pump might too.

I've heard yes and no on the pistons.
You can buy reverse rotation cam, gear drive, etc from merc. But I think the only ones still available are the 340hp inboard cams. Also not sure if these parts can be used on Gen V or VI engines.
Definitely a new rear seal.
Some recirculating water pumps are bidirectional, you'll need to check your current one.
Simple stuff, change the firing order, and swap the inlet/outlet of your fresh water pump.

I'm In The Process Of Building A Reverse Rotation Engine For My Boat To Run Arneson Asd-8 Drives. Need To Change Gam, Starter, Sea Pump.