Easy: Attach the input from the camera (the contacts on the hotshoe will just be closed) to a timer (eg NE555) or a delaying resistor/capacitor circuit and have then your flashgun triggered. You will need a circuit to drive your modern flash, but those can be found.
Not so easy: Finding the proper delay time. As your old camera most likely has a flash-sync speed of 1/60 or 1/100 second you could start out with 5-10 ms and go from there. If you're camera still uses film, expect to waste a roll on test shots to find the proper delay.
Korman