Sewing a new patch onto the crazy quilt of foreign mercenaries being spread across the Arabian peninsula, North Yemen has hired a dozen Taiwanese pilots for its air force to help deter new attacks from South Yemen.

In becoming the Cubans of North Yemen, the Taiwanese square off against the Cubans of South Yemen, who happen to be real Cubans. The lengthy border conflict between the two poverty-striken, misnamed Yemens now involves the prestige of Saudi Arabia, the United States, Ethiopia, East Germany and the Soviet Union.

The volunteers in neighboring countries ready to do the locals' fighting for them if it comes to that, include British and Pakistani officers in both Oman and the United Arab Emirates.

Across the Bab el Mendab strait from South Yemen, several thousand French troops watch warily over Djibouti's security while 18,000 Cuban soldiers and some ranking Russian generals do the same for Ethiopia, which in turn has dispatched some of its officers to help out South Yemen and, much farther south, the Rhodesian guerrillas.

Tension has become a principal growth industry for the Arabian peninsula since the 1973 oil price hike and embargo focused world attention on Saudi Arabia and other oil-producing nations, which have become entangled in the network of security subcontracting.

Taiwan's entry into that network stems from border clashes between the two Yemens last February and President Carter's decision to rush a squadron of F5E jet fighters and other equipment to North Yemen as a response to the fighting, which ended in early March.

In authorizing the first presidential waiver ever used to send arms to a purchaser without full congressional review, Carter got the 12 warplanes to North Yemen six weeks ahead of the first scheduled delivery, and a long time before 12 North Yemenis would be able to fly them.

The speedy delivery was designed more to claim an alarmed Saudi Arabia than to turn the tide of the topsyturvy fighting between the U.S.-assisted Yemen, which is actually named the Yemen Arab Republic, and the East bloc's Yemen, which is actually the People's Democratic Republic of Yemen.

The two Yemens agree that there should be only one Yemen, but have been fighting with each other spasmodically for a decade over who should run it. Their tribal societies are improverished and have to depend on outside aid and worker remittances.

Saudi Arabia is North Yemen's principal bankroller and agreed to pay for the F5 aircraft as part of a major military expansion there to counter the steady buildup of Soviet and Cuban advisers and weapons in South Yemen. U.S. intelligence reports say Cubans fly air defense missions for South Yemen.

The accelerated delivery of the F5s touched off a scramble by the Saudis to recruit foreign pilots to man the planes until North Yemen has its own fully qualified pilots. Taiwan's brand of anticommunism has long impressed the Saudis, who have guaranteed low-price oil shipments to Taiwan since 1973. The Nationalist Chinese coproduce and fly the F5 at home.

Asia experts in the State Department said recently that Taiwan apparently has no other military personnel stationed abroad and said they could provide no details of the agreement with North Yemen.