say you had bought a late 1980s VFR400. Japanese law mandated a set top speed. To achieve this, Honda put a device into the speedometer to prevent the bike exceeding this speed. Fixing this is difficult. If you take the device out, the bikes ECU ("brain") thinks there is a fault and the bike won't work. The solution is to buy a purpose built speedometer and ECU - mainly these come from the racing world, but with a hefty price.
For the DNA, I can imagine two possibilities. If you have a manual, look for where the air-intake is for the engine. This will look like a short tube that is wider than it is long. If it has a metal disc with a hole in it (washer) surronded by some glue-like sealent this is the restrctor. Merely remove it to regain full power. Or to restrict, insert such a washer - a dealer should be able to adivse, its not a big job.
Alternatively, the restrictor will be in the exhaust system somewhere. This will require a new exhaust system (an aftermarket, or official "performance" varient) to fix.
Finally, if your question is on how to make a 50cc bike go over 30mph...you have a couple of scenarios:
If the manufacturer has limited it to 30mph by an electronic mechanism, you will have to buy an speedo or ECU that gets around this. Try eBay and a dealership. If it is that the bike is set out to not get above 30, you can tune it to beat this. Options inlcude a new air filter and race exhaust - illegal for noise laws mind - and a rejetting of the carbs to suit the new system. Altering the rear sprocket will add top speed at the expense of accelleration if the gearing is desgned to keep it at 30 (this is less likely a plan by the manufacturer). Fianlly, you are looking at engine tuning such as increasing the egine size to 80cc - you're better off getting the bigger cc model in the firstplace then.