I’m not a lawyer, but you should speak to attorney in California, where the original company and directors are headquartered. It sounds like maybe a transaction is occurring / has occurred though? (“the buyer in EU”).
You could probably start with one of the bigger firms that deal with startups (e.g., Fenwick and West, Wilson Sonsini, etc.) just to get someone there to recommend someone else. Or, since this is HN start with ‘grellas [1].