Motorised bicycle? You can buy a conversion kit from ebay or similar, which provides you with an engine, carbie, exhaust, cables, wiring, sprockets, fuel tank and a whole bunch of clamps to bolt it all to a bicycle of your choice. These kits are universal in nature so it helps to have some mechanical knowledge, and note that some bikes with thick frame down tubes may ne be suitable for conversion. If you can afford it, get a four-stroke kit. The two-strokes are often cheaper but very crude.