A hydrofoil?
When? Where? How? !!!
That's an awful long way for a hydrofoil . . .
They haven't even managed to get the seaplane on this route yet!

You can get from Fiskardo in Kefalonia to Nidri in Lefkas and then drive to Corfu. So you could do the reverse.
Or you can get an Olympic airways plane Corfu-Kefalonia (2 days a week). Or the reverse.
Or I once spent the whole day getting from Corfu to Kefalonia via boat to Igoumenitsa/bus to Patras/boat to Kefalonia because there was no other option on that particular day. So you could do the reverse.
In the summer the Italian boats occasionally call in at both Corfu and Kefalonia on the way to Patras. So occasionally they will do the reverse.

It's actually easier to get to Albania from Corfu than it is to Kefalonia!

Our daughter and son in law got a day trip to Kefalonia from Zakynthosbut it was an organised trip only sore certain parts of Kefalonia which they loved, but never known of any trips from Corfu.
pojo

there is no hydrofoil. there is an inter ionian flight that goes twice a week, bookable through olympic airways. otherwise, you have to go over to the mainland, get a bus from igoumenitsa to kilini and then the ferry across. it is a right pain.

There's a hydrophoil to Paxos. Pretty certain there's not one to Kef.

The Hydrofoil hasnt run for a couple of years now ( it was a 3 hour bumpy ride).
You can fly with Olympic.
You can now also fly on the seaplane Airsealines