MHE0010500 Kit,'02,Heater,HotWater,NoRota 27  4  377.35  $15.00    is a complete kit, 
choice one : Remove core and blow into water tube with hand on other end if it leaks air its bad. 
replace bad core: 
Choice 2 find leak and repair( I wouldnt after 10 years) 
Choice 3 remove heater block off 2 water hoses. 
Capt Rick