But Leopold I never converted to Catholicism. I remember reading that on his deathbed, when his daughter-in-law, Henriette, tried to get him to take Catholic last rites he refused (in German, too). Whereas conversion would be a key point if he were to take the throne of a formerly Spanish, Catholic country.
I think you would do well to find a Spanish infante or a Neapolitan prince. There were enough lying around.
1) Ferdinand VII of Spain
2) Don Carlos, (Carlos V)
3) Don Francisco de Paula
4) Don Sebastian of Spain & Portugal (nephew of the King of Portugal, cousin to the King of Spain)
5) Ferdinand II of the Two Sicilies
6) Carlo Ferdinando of Naples, Prince of Capua
7) Leopoldo, Prince of Salerno*
8) Louis II of Etruria (Carlo II of Parma)
*The rest of the Neapolitan princes are too young - i.e. would require a regency. Though that might not be a truly terrible idea, since the Spanish infantes were extremely reactionary, Don Carlos was said to be "more Catholic than the Pope, more royalist than the king", and a regency might allow them to be indoctrinated with Mexican - rather than Spanish - sympathies and lean more to the constitutional monarchy than the absolute.
There are random Catholic princes who might be persuaded to take the throne - Saxony, Bavaria, Savoy-Carignano (unlikely), de Beauharnais (would be interesting since he was proposed as a candidate for the Belgian throne but lost to Leopold, and his sister was the Empress of Brazil and his brother the king of Portugal).
But many of the princes asked might refuse on various grounds, leading to someone saying (much like after Queen Isabel II was deposed) "to find a Catholic prince in Europe is like finding an atheist in Heaven"
Also, no Hanoverians are going to convert to Catholicism for a crown that might fall off their heads as easily as it fell on, especially when there's a possibility of inheriting Britain, and when the Act of Settlement (1701?) prohibits the conversion and marriage to Catholics. Bear in mind, Victoria was a child of three, most of her cousins not much older. The Duke of Cumberland was reactionary and unpopular, Cambridge was serving as viceroy in Hanover. Though maybe the Duke of Sussex - he was already married (though in contravention of the royal marriages act) with two children (who were regarded as little better than illegitimate).