A basic 65-70cc cylinder kit, a needle bearing crankshaft, an exhaust, a 14mm bing carburetor, and a 16-17 tooth front sprocket will get you there! A basic 45mph e50 kitted maxi is one of the most common setups out there, and for good reason. There's tons of aftermarket, and they're easy to work on and reliable. Add new tires and cables to your list too!
If you tell us a bit more about the type of riding you'll be doing, making accurate recommendations will be alot easier. Rider weight, riding environment, and budget would help establish a baseline.
In the meantime, scroll up and click "Flat View" if you haven't already, and do some reading in the wiki! There's a detailed guide on rebuilding e50 engines in there.
And post some pictures of the bike!