what type of 49cc engine? 2 or 4 stroke? Is the engine a honda clone type? If so they make aftermarket conversion kits to convert to 4 speed manual but it could get pretty involve and expensive. You might be better off to shop ebay for a larger used engine that is already a 4 speed manual. If you want to use what you have, I would just fab up a linkage for the shifter to the front. As for the drive system the most straight forward way would be a chain from the engine to a rear sprocket. More info on what you have and what you are trying to do would be needed for any useful suggestions.
