I would expect this to be officially settled before the embargo is actually lifted. Ownership of nationalised industries and other US assets has always been item #2 on the diplomatic agenda, with #1 being military alliances (originally with the USSR, now Venezuela and China).
I expect the Cuban government will concede on a few national-level items (like legitimacy of the Guantanamo base) in exchange for the US government publicly affirming that any ownership claim from US businesses and individuals pre-revolution will be considered null and void. Anything else would be complete madness. Apart from difficulty in tracking original documents (which were likely destroyed during and after the revolution), handing nationalised assets to US citizens would mine the economic power base of the ruling elite in Cuba.
Consider hotels: they power one of the few sizeable economic activities on the island, i.e. tourism; but any hotel built before the revolution (and there are quite a few, all around the island) would have to revert to (likely US) previous owners, instantly transferring a lot of wealth out of the island. Not gonna happen.
Exiles who left the country. If you were to speak to elderly Cubans who fled they would tell you about how they all had their possessions taken from them.
Any reason to expect they have a stronger or more likely to succeed claim than, say, the Japanese-American families whose possessions were taken from them in WW2?
I guess this will partly depend on the sentiment of the public. If you look at retributions for damage in World War Two, one sees that even cases where people _sold_ items to German citizens (possibly under duress, but after decades, and with many archives destroyed in the war, that is hard to prove in some cases) that were settled in the fifties or sixties got reopened decades later.
Longer-term, I also expect fierce legal battles over the ownership of houses on Cuba.