"had a decisive" is correct and usable in written English.
You can use it to emphasize the importance or impact of something. For example, "The final debate had a decisive effect on the election results."
Nothing could bring Evans back from the dead, but the case and the book had a decisive impact on Britain's decision to abolish the death penalty in 1964.
Had this conference happened even a month ago it would have had a decisive impact, but the time may now be past.
Pareja had a decisive game.
"My election had a decisive impact.
If Hunt had played 32 Qb7, he would have had a decisive edge.
For all their brilliant football, City – like QPR back then – have never had a decisive lead.
These moves had a decisive impact on the coalition's response to the widening insurgency.
A century later, his ideas had a decisive influence on America's founding fathers.
These typologies have had a decisive impact on the development of subsequent, more specialized sociological inquiries.
The relation with Fanny had a decisive effect on Keats's development.
Fischer's country house designs had a decisive influence on the architects of his time.
