String reversal function
main should be int main()
Why C-style strings? Use std::string
One function - one task. You are mixing 2 things into one function:


Reverse chars in a word 
Find words in a sentence separated by spaces




I feel like your problem with too many variables will resolve when you split the tasks you are doing into separate functions.
Use better code formatting (auto-format).
Type mixing - strlen's return value is std::size_t. Do not assign to int. As a follow-up, use a loop index of type std::size_t to match.
This is my preference - not using using namespace std; and system("pause");. I think a lot of posts could be found explaining why.