Type of Cherries for the Topping?

If you can only get sweeter fresh cherries (so wonderful this time of year!), you could use those, but add a touch of lemon juice toward the end of the cooking time, cooking it down to reduce as necessary. You could do the same with frozen cherries that aren't sour cherries. It won't be the same, but it will certainly be delicious. ;o)

Tart/sour cherries as rec by recipe. Great tasting but available fresh only a few weeks in summer.
If you want fresh but can't get tart, use other (still good but not tart) cherries.
If you want tart, use year round available - canned, dried, frozen.
If using canned, drain and use.
If dried, rehydrate, drain & use.
If frozen, thaw, drain & use.