Callum,
Look into direct-coupled electric actuators by Belimo, Johnson Controls, Honeywell, Kele. They're made for dampers and are easy to set up for 90-degree rotation. Pretty inexpensive too. Torques up to 300 in-lb. For higher power, industrial valve actuators are available, or you can stack the direct-coupled ones together on a shaft.
Best to ya,
Old Dave