Do more than the impeller - get a waterpump "kit" - it will contain some other seals and gaskets as well as the impeller.
There are 3 different levels of "kit" available - unless you are absolutely certain of the history of the motor, I would spend the 80 bucks on the kit that includes a new base as well as the pump cover (along with the gaskets, seals and impeller) - then you will know for sure.
And if you are cringing at the price, that is cheap insurance against frying a powerhead.
This motor did not ship from factory with a thermostat but they always run better if you install one (merc probably save 50 cents a motor by not including them).
A thermostat and gasket is about $25 at any Merc dealer.