Can't solve the problem itself, but others here will be able to offer you electrical solutions.
Top speed, increase by 3 or so mph on medium or high power WITH pedal input, so where electric assistance cuts at about 15 mph, it'll carry on assisting to 18 mph. Hardly record-breaking stuff. Higher speeds than that, it's down to how hard and fast you turn those pedals.
I don't THINK it affects range very much, as it's effectively only a speed restrictor, but again, electrical experts will give you more definitive answers.
As for getting it done if you don't do it yourself, depends where you are. If you live anywhere near the main Wisper dealers, give them a ring as I'm sure they could sort it for not much money.
A.