Monopoly: The Mega Edition Bankruptcy rule for distribuiting properties of creditor
Yes, you must un-mortgage the property and redistribute the building across all the properties.
Rules

Owning all but one


  You may build houses and hotels once you
  own all but one property in a color group


Accruing the last one


  If you acquire the
  final property of a group, after previously building on its other
  properties, you must un-mortgage this property immediately
  (if mortgaged) and build it up according to the “even” building rule or, if you can’t afford to do so, you must redistribute
  existing houses among all properties of the group. You may
  need to break down hotels to do this