if you want 5-10mph more ... then a 70cc is overkill (and could ahem kill your engine).
now that you have a biturbo ... get a larger manifold and a larger carburetor. what kind of bike do you have? if you have a dellorto carburetor, you can see if you can get a 15.15 or at least a 14.14. also you can play w/ jets to see what works best.
another option (on top of these things) is to "port and polish" so that your cylinder has larger ports. that should get you more speed.
then there's also changing to a larger front sprocket (or smaller back sprocket). you'll loose acceleration, but gain top speed.
i have a 50cc w/ normal cylinder ports and my bianchi goes over 40mph. it has a dellorto 15.15 (w/ #53 jet), 11-tooth front sprocket, and a polini performance exaust. all these upgrades (including the exhaust) cost me about $150. so it's cheaper than a 70cc kit.