Paigeoliver,
That's funny that you are going to be putting a bigger engine your your c70 (I'm guessing you're going with either the Lifan or the Zongshen, I hear the Lifan are the better of the 2 honda knock-offs).
I actually just bought a bigger engine for my 1981 C70 (it's an actual honda engine from the CT bike, it's 110cc, should be a fairly easy swap). It's still being shipped to me, I can't wait to get it and slap it on.
On one of the Yahoo groups, someone there put a 100cc Lifan engine on their bike and had it up to 50MPH no problem (and that was without pushing it all the way since the engine wasn't broken in yet). I'm not looking to drive mine on the freeway or anything either, just trying to keep up with traffic a little better (the road from my house to downtown is 45, just out of reach of my current 70cc's, as I top out at 35-40mph).
My wife is really excited about this bike too, she actually went and got the motorcycle hand book from the DMV yesterday, and she wants to get her motorcycle license so she can use this to run errands around town. These are great bikes, good luck with your upgrade, you'll have to tell me how it goes.