edb wrote:
Mastercut 13.5 HP 42 in. cut about 7 years old.
The belt broke this afternoon.
Questions:
Is it hard to replace with normal household tools?
Where does one get a replacement?
If negative to all of the above, where is a good place to buy a replacement
mower?

It is fairly easy to replace the belt. Take th old one to a mower
repair place and see if they can find one that matches. Before
replacing the mower, ask a mower repair place to come over and evaluate
the situation.