Remove the IC engine. Install an electric motor and all the necessary electronics: batteries, control circuitry, charging circuitry, etc.
You can find companies selling conversion kits for certain cars. For example, here’s one for a 1970’s VW Super Beetle.
If you are into bigger custom modifications, Chevrolet sells an eCrate engine kit. It’s basically the drivetrain and battery pack from the Chevy Bolt. You have to figure out how to package it in your car of choice. As a demonstration, they used it to convert a 1977 Chevy Blazer.
Edit: Note that removing the IC engine means you also lose heat, air conditioning, power brakes, and power steering. Those systems all rely on waste heat, belts, or vacuum lines from the IC engine. You need to replace them all with electric devices.
We’re expecting all the drivetrain components to hold up to instant torque?
That heavy torque is one reason to keep the car’s gearbox rather than go with direct drive. Chevy’s eCrate kit specifies that it’s intended for use with an automatic transmission, not direct drive.
When I’ve mulled over this question, placement of the batteries is usually the sticking point. Electric cars are designed to put the heavy batteries in a relatively advantageous spot for handling.
There’s a company in my town that electrifies vintage European cars. I’m told that the bulk of the work is figuring out where to put the batteries and how to cool them.
