I have old Marui Hunter thats been laying around years, with broken idler gear (common problem, stock idler gear doesnt have bearings).

Sooo... I designed a improved replacement (OEM part nr. M0134)

This design incorporates 2 8x3x3mm bearings. The gear center hole is still a 3,1mm, so you can test fit it with the idler axle without bearings, but i dont recommend driving the car without those bearings.

This gear is also compatible with Marui Galaxy.

Turn it 90 degrees in your slicer and print with supports on print plate only. Print it with small nozzle to get best results. The example that i have in the pictures is printed with 0,4mm nozzle, which is not ideal for gear like this. You should also consider using professional 3D print services to print this gear.