What is the ruling on prayer and fasting after nocturnal emission in the late night, and performing ghusl, but not having a spare pair of underpants to change into?

Fasting does not require purity from ritual impurity (hadath (definition)) or physical impurity (khabath). Fasting is valid while in a state of janabah (definition) (major ritual impurity) or with impure clothes. This is supported by the fact that the Prophet, peace and blessings be upon him, would wake up in a state of janabah due to intercourse, then perform ghusl (definition) (ritual bath), and fast. As for prayer, it is not valid without purity. You have become pure from ritual impurity after performing ghusl. The preponderant opinion is that semen is pure, but it is better to wash off whatever semen has touched one's clothes to avoid the differing opinion of those who consider it impure.