Is the Niyyat correct if one might need to think and it may take some time before the mind recalls which prayer one is praying? Do we need to actively keep repeating the intention in our mind so that we don't forget?
We do not need to keep repeating the intention (Niyyat) as far as we still remember what we are intending to perform. If we forget what want to perform, then we need to remember the Niyyat again.
'Niyyat is nothing more than to know what you are going to perform, and it is for the sake of Allah. We do not need to say anything to reflect our intention.
Wassalam.