.A 1980 S76, is it really economically viable to rebuild it?
No, probably not.
A large operator recently "traded" their remaining A+s on a deal for Ds but not all are in flying condition and will probably go for scrap.
Even the C+ XXEA which was taken in trade for XXEB wasn't sold last time I checked (which was a while ago) and could have been bought for $3m. Sikorsky also had a few other corporate low time C+s for sale not that long ago.
I know of a fully re furbished low time EMS configured C which has been for sale for 4 years.
There are at least 3 offshore configured A++s in Australia for sale but they are past their use by date for the oil and gas industry.