Here is a listing, from Marine Diesel Direct, of some of the common parts for the Yanmar 2GM20F (note: I now use Gates water pump and alternator belts and feel they are much better quality than Yanmar)

See: www.marinedieseldirect.com/yanmar/shopping/engine.php?engine=2GM20F

128270-12540 Air Filter
124060-59380 Bleeder Plug
23418-060000 Bleeder Plug Washer
104500-55710 Fuel Filter
24341-000440 Fuel Filter O-Ring
128296-42070 Impeller
119660-35150 Oil Filter
104511-78780 Water Pump Belt
124223-42110 Water Pump Gasket
124223-42080 Water Pump Seal

Horsepower: 18
Produced from: 1983 to Present
Series: GM/HM Bored Up series
About the GM/HM Bored Up series: Similar to the earlier standard GM/HM series, these engines provide upgraded horsepower and a 55 AMP alternator (1GM10 35 AMP). Special Note: If your engine serial number starts with an E you must make sure that you use parts for the YEU engines. For example, if you have a 2GM20 with a serial number starting with E you actually have a 2GM20YEU. The (YEU) engines are European manufactured and now more common in the United States than the Japan built models.