I assume this is a one-off replacement. You have several options, but none are trivial...
Find a secondhand supplier. There are several such dedicated companies on the web, but they will be expensive.
See if there are 4000 series devices that have suitable timings and pinouts. Pinout could be dealt with using a carrier board.
Use a modern small outline device on carrier board, together with a voltage regulator and level shifters. Be careful of the risetime of a 5V->8V shifter; some can be good up to 5V, and anaemic from 5V to 8V
Timing: most people will verify the tPD and tS timing in their circuit, but tH is equally important and can be less tractable.