Another possibility is if an election was made in 1994 when the lifetime capital gain exemption was eliminated.  This would provide a basis for valuation.

In my opinion in lieu of definitive records the best solution is to gather as much market data relevant to the approximate periods that the shares would have been purchased.  Then calculate a reasonable estimate erring on the side of caution.  The key is be reasonable - the CRA isn't likely to spend a lot of time challenging a minor discrepancy and even if they do disagree it is unlikely that they will do more than assess interest on the outstanding tax owed.

Yes you are technically responsible for maintaining records but since that ship has sailed you need to do your best to remedy the situation.  Which is providing a reasonable estimate.