Conventional stairs take up a huge amount of space-- space you can't possibly have in a 24 ft square building.
You can't get anything but yourself up a spiral stairs, and spiral stairs are not inexpensive. They still take up a space 6 ft square.
When considering how to use the space above the shop for storing large and seldom used things in the big-house barn, I looked to old barns with haymounds for inspiration. Most of then had a ladder and small access on the gable end of the building (near the peak on an outside wall) then a large access hole with an insulated panel in the center of the space. The hay barns had a block and tackle arrangement hanging from the ridgeboard at the peak, allowing a person to lift heavy things through the large access. I had a scissor lift to use, but a simple Harbor Freight winch would lift everything you needed up into the "haymound".
I'd recommend a short extension ladder for the "man hole" access, rather than a straight vertical ladder on the wall, but either will work. Both access holes (man hole and cargo hole) can be drop-in insulated panels. This is much, much less expensive than any kind of staircase, and allows very large things (seats, spare engine shrouds, etc.) to be stored "up".
Whatever you build, plan on maximizing every square inch of the space, because you'll need it.