Cool - I hope it goes well.
I was interested enough in the subject to create a prototype design for the project. (Retirement gives you more time to play). Your Photo was helpful for the major parameters, and I just estimated the remainder to produce this in Fusion 360. Your spur gear looks like it may have 15 teeth, so with that as a starter, I applied 105 teeth to the second gear, giving a 7 to 1 reduction at the required dimension. The relief holes in the larger gear are for weight reduction and saving material.
The colours you see in my image are simply selected to highlight the two gears meshing.
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