NEVs are usually a specific class of vehicle that is legally limited to certain places, speeds, etc. You'd have to check your local and state laws to see if there is a way to convert one into a different class of vehicle.
Since they're meant for low speeds and short distances, they may not have the same crash protections, brakes, etc., and so may not be convert-able without adding all of the other requirements for the other vehicle class you want to convert them to.
If it can be legally done, then you could physically upgrade it to go faster. It's probably limited to 25mph, and likely only designed with enough power to do that. To go faster for short bursts you could simply change the controller and battery for one that is high enough voltage to turn the motor at the necessary RPM, but it takes a whole lot more power to go 55 than 25 (see the ebikes.ca motor simulator for how this works and what power it would take to go that speed).
If you need that power / speed long enough the original motor (probably a brushed motor like those in forklifts) would probably overheat without some extra cooling, and possibly even with it, and you'd need to change it out. But that probably costs less than changing the controller and battery by quite a lot, since motors are a lot easier to find used in working/adaptable condition than batteries or controllers.
Since it's possibly a brushed motor (depends on the year), it has a physical RPM limit that if exceeded coudl cause the commutator to "grenade", throwing segments out into or thru the motor casing into the rest of the vehicle, etc. So you might have to change the gearing ratio to the wheels instead of the actual motor RPM; something you'd have to look into once you can see what the motor nameplate shows, etc.
Keep in mind that the motor usually drives a transmission, and may also drive pumps, etc., so running it at more than twice the original RPM may cause a problem with those devices.
Batteries you could get from places like BatteryHookup, or other used-EV-battery-sellers, and build a new pack for it from that.
Here's one thread on this type of vehicle with some pics, etc:
Hi all. I'm just getting into my latest project...the re-assembly of a Miles ZX40ST electric truck. Here a link to the album on Photobucket...more pictures will come as they are taken. http://s935.photobucket.com/user/mollysplayhouse/library/Miles%20ZX40ST%20Truck%20Restoration?sort=3&page=1...
endless-sphere.com