The Isfd instrument failure question as posted is bit vague.
I would think you have more chance to have a IRS failure in flight than a loss of all normal AC power(Dual gen failure).
For the "HDG part of the Isfd" to be inop you would need the L IRS (on my fleet) to fail.But the right IRS will provide correct heading info to the right PFD.
In that case as i mentioned the APs wont be available for the flight and if you are in for a long flight id return..
For the "STBY ATTITUDE part of Isfd" you d have to have no AC power at all..as it will work with battery power via the battery bus.(if both gens fail and dipatch with APU inop).
Then we all know we need to get on the ground asap.
If you lose the RMI (ac stby bus powered..), so you have no more AC/DC power, well yes you d need to fly with the magnetic compass for headings..
I would not take off with ANY standby instruments INOP.
As far as i remember flying with the mag compass, during the turn you need to overshoot South and undershoot north.
There are some acceleration errors but i forgot...(maybe flying east and west?)