get a set of genuine HTK brand glow plugs, easily available at Plaza market, be sure to match your old plugs voltage to the new ones,
If your old plugs are very carboned up buy a drill bit about the same size of the tip of the glow plug, cut up an old dead glow plug and weld the drill bit onto it so its about the same size as the glow plug, use some grease on the bit and ream out the glow plug holes to cut the old carbon out, it will greatly increase plug life and ease start up. crank the engine over a few revs to blow out the carbon dust and install new plugs. be sure the bus bar for the plugs has very clean contact faces for the plugs connection nuts.