Where is the block heater plug on a 1990 Honda Civic SI?

The 1990 Honda Civic Si does not have a factory-installed block heater. These weren't standard equipment on that model, especially not in areas with milder climates. Any block heater you find would be an aftermarket addition installed by a previous owner. If one exists, its location would be entirely dependent on where that owner chose to install it. You'd have to visually inspect the engine block and surrounding areas to locate it, if present.