SInce the drivetrain is gone, painting your car original color will not increase the car's value.

Here's why.

People who collect/invest in these cars are concerned about completeness and correctness and correct color. You also have a market for non original drivetrain and colors. You have a fairly common color and an uncommon/unappealable color for a non numbers matching car. If your car had the original drivetrain then yes....paint it original.

You would be better off painting it a color that would appeal to a broad spectrum of potential buyers and be period correct....maybe a silver. Black is too hard/expensive to repaint unless body is already razor straight.

In other words...Mayfair Maize is not a color that appeals to people looking for an investment with a non-original drivetrain.