Electric (small) dirt bike and torque

Hello everybody !

I modified a “Razor MX650” : put a brushless 1000W motor (1800W max) and changed the acid 36V battery by a 48V LiFePo4 one.
That’s was working great (30 miles/h=50km/h) except … razor is much more near a gadget than a real motorcycle !
So I bought a Kawasaki Kx65 and put battery and motor on.
That’s works too … except no torque and speed too high, even I put maximum gear ratio (48:12).
So this is the reason I’m here: I’m searching better method to reduce ratio (I don’t need speed and reducing by 2 or 3 will be correct. I just need best torque)
Actualy I’m searching for a “planet gear” reductor who be interesting because not adding gears & chain.
Have you good idears for this ? others ?
Thanks and sorry for my bad english.

Cant you just reduce the gear ratio? Smaller pinion/larger spur?