What units and structures in Starcraft can be repaired? Can allied units from other races be repaired?
I just found this which answers all questions!
SCVs can repair Terran buildings or mechanical units (including other SCVs) under attack which is a key part of defending early game pushes.
Basically put: SCVs can repair any Terran structures or any mechanical units. "any" isn't in the original quote but I want to put emphasis on the fact it does not narrow down mechanical units to a specific race.
Of course they can only ever repair allied units. As a side note, Zerg does not have any mechanical units and if he Neural Parasite'd one the unit would still be Terran or Protoss.