I have a Honda Foreman ATV (450cc) that is awesome with loads of power. Honda and several other ATV manufacturers are now making single cylinder 4-stroke ATVs at 650ccs. Only recently have ATVs become available as twin cylinders (Kawasaki Prairie - 650cc).

I have an old Ryobi 4-stroke trimmer with inter-changeable attachments that supposedly had the smallest commercially available 4-stroke engine. I'm not sure of the exact displacement, but it's pretty damn small. It has several teeny valves and an oil pan that holds several tablespoons worth of oil. It's been worked pretty hard for eight years and still going strong. Best thing about it is the lack of 2-stroke stench exhaust!