heck if its just a plain old hot water heater.
That's simple.
cut pipes
toss out old system.
get copper tube, flux, solder,torch, fittings
sweat pipes. ( put on flux, torch it, put fitting on, solder with torch) ( I like MAP Gas, Yellow bottle burns hotter)
fill system up. Check for leaks with dish soap and H20 in spray bottle.
any bubbles re solder.