Since TTL's Franz Josef is the same age as his OTL cousin, unless he's married off before 18 or we butterfly away his uncle Ferdinand's abdication, he likely will still be emperor at the time of his wedding.
He might meet Helen and Elisabeth, as they are his cousins, but likely won't marry them. If he needs a maternal cousin for a bride the Saxon branch (either a daughter between Frederick Augustus and Sophie or one of the surviving daughters of John and Amalie Auguste) will be more prestigious matches. Otherwise, you're likely looking at an Italian princess, a Habsburg cousin like Archduchess Maria Isabella, a French princess (looking at your list of kids for Charles X Princess Louise Elisabeth would make the most sense), a Spanish Infanta (likely not the daughter of Maria Josepha Amalia, but instead a daughter of Maria Cristina), or a Brazilian princess from a second marriage of Pedro I.
As such reasonable brides for Franz Josef (based on OTL and already discussed ATL princesses)
- Princess Louise Elisabeth of France (b.1834)
- Infanta Isabel of Spain (b.1832)
- Infanta Cristina of Spain (b.1833)
- Daughter of Pedro I through remarriage
- Duchess Helene in Bavaria (b. 1834)
- Archduchess Maria Isabella of Austria (b.1834)
- Archduchess Maria Teresa of Austria (b.1836)
- Archduchess Marie Henriette of Austria (b. 1836)
- Archduchess Elizabeth Franziska of Austria (b. 1831)
- Princess Maria Elisabeth of Saxony (b.1830)
- Princess Maria Sidonia of Saxony (b.1834)
- Princess Anna of Saxony (b.1836)
- Daughter of Frederick Augustus of Saxony and Sophie
- Italian Princesses I've currently not thought of
- ATL sisters of any Catholic royal born between (1830- 1836)