Hi!
Gramatically we have two possibilities:
(1) FEL belongs to TEMP, or
(2) FEL belongs to REPARATIO,
where TEMP could be singular or plural.
(1) would be FELICIS TEMPORIS REPARATIO (singular), or FELICORUM TEMPORUM REPARATIO (plural), meaning 're-establishment of the happy time (singular)' or 're-establishment of the happy times (plural)'
(2) would be
FELIX TEMPORIS REPARATIO (sing.), or
FELIX TEMPORUM REPARATIO (plural), meaning 'the happy re-establishment of time (sing.), or 'the happy re-establishment of times (plur.)'.
I thinks most of us lean to the singular form of (1).
Best regards