Thanks for your hints for improvement.
There are some design constraints I didn´t mention. Those constraints would be obvious when you try to build it.
1.) The device already goes pretty close to one of the rivets that hold the scales. A bigger wheel could make it neccessary to redo the riveting.
2.) A bigger wheel would have to protrude more in order to spin it.
3.) The use of lighter flints was an essential design feature. It should ensure that you can buy replacements virtually everywhere.
4.) The screw that holds the flint and loads the spring has to have a core diameter of at least 2.5 mm (the diameter of the flints). M3 happens to have that core diameter. If you take a M3 headless slotted screw it can be driven with the small eyeglass screw driver.

The screws that are visible are M3 counter sunk. I used a counter sunk head screw for holding the sparker because the slot is less prone for damage and could still be unscrewed if the slot is slightly damaged. A headless slotted screw could be virtually impossible to remove if the slot or thread is damaged.

In my location I have no supplier for the Spark-lite. So it wasn´t an option for me.
Apart from that I wanted to keep the costs low as this was just a feasibility study.
I found that the sparking wheel of a BIC lighter doesn´t show signs of wear after the lighter is empty. The design allows fairly easy change of the wheel. So I don´t see a real problem with this.

Maybe the members who try to modify their SAK with a sparking wheel could give a short report on their solution. I´d be interested in that. When the flaws of the actual designs are ironed out, the sparking wheel on the SAK may even become a real preparedness item.