Looking for a non existant part!

I bought a couple of motor gearheads to up my torque and lower the RPM at the axle, I was expecting to have to manufacture a plate to mount the gearhead on the motor but was also expecting that the 8mm motor shaft would simply 'fit' the 8mm drive hole on the gearhead - it doesn't! - after a bit of net trawling I found that the gearhead expects the motor shaft to be a helical cut spline on the motor shaft that mates with the first stage of gears in the gearhead, after many more hours of net searching I am unable to find either an 8mm OD pinion shaft or any other type of solution that will allow my standard 8mm motor shaft to drive the gearhead. HELP! Any ideas greatly appreciated apart from "go and machine one", whilst I have found dozens of spec docs relating to this and I now know that 4GN describes the required standard pinion I cant find anything that says what the 4GN mod depth should be or how many teeth should be on it!

https://1drv.ms/v/s!Ao-qA_q7UGcplDmXLAhN2NER7IBe

The only solution that is "acceptable" is to find out what motor should fit on there.

If it's a motor with 8mm shaft with cut-in helical gear, it's a brand-specific motor.

But I can tell you off the cuff that it will be a heavy, but powerless motor, whatever weightclass it fits in on size or weight.