Amtrol Boiler Mate Hot Water Maker WHS-120 thermostat replacement or new unit?

Owing to hot water not being hot enough, I've been quoted $1,400 to replace my old analog Amtrol boilermate thermostat with a digital thermostat. 
They said they couldn't get the analog replacement but it looks like it's available here. 
Amtrol - Aquastat/Thermostat, 2704-093

(I wondered if it would be cheaper to replace the whole tank since it is 20 years old, but apparently not. Please verify.)
Amtrol Boiler Mate Hot Water Maker WHS-120. Boiler is a Viessmann EDC-155.

Quote: Remove existing Amtrol boilermate 120 thermostat and replace with Tekmar 150 one stage setpoint control (CRT150) and Tekmar 071 universal sensor (CRT071).

Advise please. Thank you.