Is that cupola for looks, or does it vent?
If you already have gable vents, one thing that will really help is to put some additional vents in the walls down low. I used to work in a factory that made sheds like that, and delivered and installed dozens of them (we used a Jerr-Dan roll-back car hauler). When I build my own shed at my last house, I had gable vents, but also put a couple vents in the front and back walls, thinking it would help cool air come in and improve airflow out the upper gable vents. That shed never got stifling hot, and one time I was in there I noticed cobwebs wiggling in the breeze from airflow coming in the lower vents. So they did seem to help.
I never had issues with equipment (Deere riding mower, Honda push mower, wed whacker, power washer, etc) or fuel or chemicals due to heat. At least not any issues that I could attribute to heat.