Motorising a bicycle.

Buy a petrol kit off ebay and follow the instructions. At that point you fall in a gap between cyclists and bikers, where neither group wants to know about you.

Or buy a new bike designed as an ebike. Or buy a nifty-50 scooter/step through motorbike.

An electric assist motor is a better idea, the kits cost about the same. Main difference is you're consuming power to charge, instead of burning petrol+oil.

Electric is much quieter too, whereas motorised bicycles are incredibly loud because of the small mufflers.

Electric kits tend to replace your front wheel, and have a battery bag that hangs on the frame or goes on a rear carrier, or inside a pannier or basket.

Good thing about electric is you're still a cyclist.

Here's my adventures with such a kit http://www.electric-bike-kit-forum.com/post2249.html

The way it was done when I was a kid was to find a small horizontal shaft engine (large chain saw, or some mowers and tillers), attach a drum (roughly 2" diameter) to the shaft, then mount the engine on a board which was somehow suspended off one side of the rear wheel with a mechanism to raise and lower it.

When the engine was started and the drum lowered onto the wheel, the bicycle would move. If you remembered to mount the engine on the correct side of the bike, the bike would move forward.