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