The EL-13 'electronic' flasher is quite a bit different inside than the usual 'thermal' flasher. It's more or less a relay with two windings, and a capacitor.
Unfortunately, I don't recall just what I did overall; it was kind of a trial and error process. I do recall that the margin between "faster" and "not working" was very small.
I don't particularly recommend the mod to others, just reporting what I did. If I manage to find an original Lucas can, I'll probably try to stuff a 'real' electronic flasher inside. NE555 anyone?