45 is doable on a 49cc.
The best place to start is with a big bore kit. 150cc engine can be installed too. That would get you to 55 easily.
100cc kit. Everything you need.
http://cgi.ebay.com/ebaymotors/100cc...-/330459445932
You will want to up the carb and exhaust too. 100cc should get you to 45mph.