I'm guessing the engine is a Briggs and Stratton. If so the spark plugs tend to be all the same for the small engines. I can't remember the part number but places like B and Q normally have them on display near the lawn mowers and accessories - there is usually only one type so it should be the correct one. If the current spark plug looks OK'ish, the problem probably lies with either the points and/or condenser fitted under the flywheel. It is not unusual for these to need a bit of attention, especially if the engine is old.
A useful tip for machines like this is to either use fresh petrol or add Briggs and Stratton 'Fuel Fit' to you petrol can, because modern fuel goes stale very quickly and can cause starting problems. However this won't cure a lack of spark!
There is a Briggs and Stratton book/CD covering repair and maintenance of these engines, and they often appear second hand on ebay, or for a bit more money you can buy a new one.
Good luck.
Matt.
October 2007