And that motor is only 180w motor..
I've had that motor on one of my son's electric bike and it sucks...
Even with 36v it only gets up to 18mph/30kmh...
You could TRY to add another 12v SLA, to make it go a little faster, but 2 caveats...
1.) Currie has been known to set limits in/on the controller to not allow more than 24v.
2.) You might actually blow the controller if you throw another 12v into it.
Problem is, if you wanted to go to another (Stronger) motor, it'll be a little hard to find at a reasonable price, because 16" wheels don't normally fit larger (250w+) motors, so you would have to get a specialized motor/wheel combo.
This one company sells a (quite) powerful motor already "laced" into a 16" wheel. (It's actually a castes wheel)
At the cost of $450 including shipping it's a bit on the high side.