Maybe this was addressed long ago, but I was wondering about something.

When replacing a selenium rectifier with a diode, how do you figure out which diode to use?

Check this out: http://wiki.answers.com/Q/Can_you_replace_a_selenium_rectifier_with_a_silico n_rectifier

I generally use 1N4007 - rated for 1000 PIV and 1 Amp.

That's what I use, too. Often, a resistor has to be placed in series with the diode to drop the excess voltage.