Motorising a bicycle. Can anyone advise on the best way to motorise a bicycle apart from electric motors? I think I would like to fit a small two stroke engine.
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