Wa`alykum as-Salamu wa Rahmatullahi wa Barakaatuh.

If you can precisely locate and identify the spot with the impurity, it is enough to remove and wash it; otherwise, one should clean the whole cloth. Aishah, the beloved wife of the Prophet, peace be upon him, says, “I used to scrape off the traces of semen from the clothes of the Prophet.”

As long as the impurity is removed, you are allowed to pray in it.

Allah Almighty knows best.