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?

Probably not, if you're mechanically inclined. Usually involves
dropping the deck (couple of clevis pins) and threading the new belt
around the pulleys and tensioner.

Where does one get a replacement?

Try WalMart. They sell MTD tractors (and parts for them). Mastercut
is owned by MTD; maybe they use the same parts.

If negative to all of the above, where is a good place to buy a replacement
mower?

You're going to buy a new mower because the belt broke?