As you know I'm doing a similar trip this summer.
On my Catalina 25 I had a 6 gallon and 3 gallon tank for motoring. The 6 gallon fit into the fuel locker, the 3 gallon fit sideways across the back of the cockpit (an otherwise useless area). Both had the same fitting, so if fuel ran out and surprised me I could just disconnect the line from one tank, connect it to the other, and restart the motor.
I carried a backup fuel line with the motor-end disconnected (but ready below) and could use that to transfer fuel from a fuel tank into my dinghy motor. That isn't important for you, but carrying a backup fuel line might be. It's one of the more likely failure points on an outboard. You can get a cheap outboard fuel tank from this store in Portland:
Sexton's Chandlery | Home
They have Yamaha-branded 3 gallon tanks piled up for $30/ea, brand new in the box. I should have bought one last time I was in Portland.
I didn't like carrying jerry cans. The modern ones are really annoying and leak fuel when you are pouring from them.
If your budget allows for it I'd add a solar setup to the boat. A 20-30w panel plus decent charge controller is about $150. On a boat with few systems like yours that will keep the battery topped up and ready to go for all purposes. This is the same system that we used on my Catalina 25, and the batteries on that boat have never been charged by anything but the motor or solar.
You might want to get the Navionics or other charts for your iPhone that work even when the iPhone doesn't have a data connection. This is the cheapest way to get a GPS for your boat.
I think you'll probably want more books.
I carry a lot of my old running rigging (anything in good, but not great, shape) because line is useful to have. There is a hard to use for anything better locker on my boat that is full of the stuff.
You might want to think about more safety gear. I think I'm going to get a harness and jacklines for when I'm single handing in rough weather.
I hope to run into you sometime on our travels. My boat is a Pearson 28 called "Elena".