Mike, the entire replacement routine requires more confidence than competence. I could do it.
Uncover the element, drain the coolant (a simple easily accessible twist valve that opens a drain tube under the AQHot), replace the element and reassemble.
The original element was unavailable so the sourced replacement may have had a higher wattage, it was the same diameter, I know it was longer so I assume more resistance and more heat. I do know that I need to be careful or I can scald myself from the water out of the tap. The recovery rate is rapid so you don't wait long for more hot water, and the furnace runs just fine without the diesel burner on. Parts are readily available for the Aqua Hot and like everything else on the market the unit is a Bitsa for the most part, I know you can purchase the March pump used to circulate the fluids for about 1/3 the price of an official AH unit.
There are repairs done by intrepid folk to the coil unit itself, I published some of those fixes here. Some competence is required but the mechanical stuff just requires time, confidence and in some cases a willing helper. After watching the repair, I now know I could do it myself and I am just a wrench, not a mechanic.