12 volt jump damage to 6 volt system?

[AnthonyG]

Just got a beautiful '35 Tudor. History, restored 25 years ago, blown engine originally thought to battery issue, owner tried to jump with a 12 volt and wife's indications to me are she thinks all electricals are blown out as a result. Original owner ( husband died 5 years ago ). My first project on 6 volt car. No engine yet.

Any thought's on the potential electrical damage from the 12 v jump?