Celia is only on the first two songs:

With Celia Cruz:
Kimbala
Guantanamera

Without Celia:
En Orbita
El Raton
Me Gente
Guantanamera
Ponte Duro
Fania Theme/Closing credits

The Line up (to the extent it is listed in the film. Notice there are no horn players listed, and there is at least one percussionist and one violin player not on this list.:
Johnny Pacheco - Director and Flute
Celia Cruz - voice
Santos Colon - voice
Cheo Feliciano - voice
Larry Harlow - piano
Felix 'Pupi' Legarreta - violin
Hector Lavoe - voice
Nicky Marrero - timbales
Ismael Miranda - voice
Ismael Quintana -voice
Roberto Roena - bongo
Jorge Santana - electric guitar on "El Raton"
Yomo Toro - cuatro
Bobby Valentin - bass

I found this text elsewhere on the 'net:

In 1974 Celia Cruz & The Fania All Stars were invited to take part in a 3-day festival in celebration of black sports and entertainment in Kinshasa, Zaire. Their original performance on the first day caused such a hysterical response from the audience that they were invited back two days later for an encore performance. The concert was part of the famous Rumble in the Jungle title fight between the reigning champion George Foreman and the greatest, Muhammad Ali. Their phenomenal performance to an audience of 80,000 people at the Stadu du Hai was shot in 35mm on sic camaras by director Leon Gast (who won an Academy Award for When We Were Kings.) Featuring Johnny Pacheco, Cheo Felicano, Hector Lavoe and Roberto Roena.