Re: Can you unbridled your 48cc engine ?
I would say $200 is a good amount. With that I would buy:
80cc Motor and and Kit from ebay, I have had good experiences with a seller called "luckyearlybird", but ask around if you like. $130, free shipping.
Walmart bicycle light set, front and rear, $30, plus batteries, $5.
Walmart bar-end Mirror, $6.
(Now we go into acessories, things that aren't necessary but make it easier to ride)
Walmart sprung cruiser seat, $25
Walmart bike bag for tools or a spare tube
Finally, if you are travelling long distances I would get a larger fuel tank, or a rear rack and an auxiliary tank.
Even getting all of that, the cost should be about $210, minus what you paid for the bicycle.
On speed, it should get around 30mph cruising, completely stock. Modifying the motor and maybe adding a smaller sprocket, among other things, could increase the speed but it requires time, money, or tools and sometimes all three. See the "high performance engine tips" section of the forum for help on speed.
Motorcycles, Motor-bicycles and bicycles- two wheels and the open air is all I need.