My old Cyclamatic went 31 mph with its original 24v motor when I ran it at 44v and 17 amps. Motors are generally under-rated, though you can damage any one by running it too slow at maximum power. As long as you keep the speed up, they can handle a lot of power.
The modern controllers, like the one I linked above use current control. That means that each level on the display controls the amount of power going to the motor, so you can turn the power up or down as you want.
If you want to use your bike like a motorbike to go 30 mph without pedalling, then you need a motorbike. 20 to 25 mph is feasible in a bicycle if you pedal quite hard., otherwise, if you go fast, the battery will go down quickly.
The Cyclotricity Stealth 1000W nearly meets your requirements. it can cruise in the mid 20s with very little effort from you, but don't expect to do more than 10 miles and expect to get a new battery before the year's up.
Pedalease do one with a 17AH battery, which is much more suitable than the Cyclotricity one for £825, which looks good value.
Don't forget that you'll have trouble trying to convice anyone that these bikes are legal. Nobody would know that your Cyclamatic had been modified.
