@jwz I've been wondering if tension can be adjusted, but I see the pulley's only supported on one side and the motor bearings probably aren't designed to take the forces. In any case, I'm sure if you can adjust it, you'll have tried it and it mustn't have worked.

In the absence of increased tension, I think the advice to use an extra wrap or two around the pulley is your best answer, but you'd need a different pulley.

I think the right way to go would be:

• Put a new, wider pulley on a separate spindle, and use two or three wraps of the cord.
• Use a couple of gear wheels to transfer the drive from the motor to the pulley spindle.
• The pulley spindle would need a suitable bearing at each end.

Benefits:
• You're no longer limited to pulleys that fit your motor.
• No more lateral forces on the motor spindle.
• If they wear, only the bearings would need replacing, not the whole motor.
• It'd cope with more tension than you need - I'm certain the limit would be the curtain rail.
• If needed, you could choose a gear ratio to better suit your motor & aesthetic effect. Otherwise 1:1 will do.

Downsides:
• More work.
• You'd still need a gear to fit the motor, but I think that will be easier.
• Probably a little noisier.
• Would need a facility to access one side of the pulley to wrap the cord round it.