Breaking a GPG signature for demonstrative purposes

["mal content" <artifact.one@xxxxxxxxxxxxxx>]

Hello.

This may sound like a phenomenally idiotic question, but I'd
like to somehow send an email to a friend with a broken
signature to make it look as if the message has been modified
in transit. I'm teaching him the ins and outs of PGP and he'd
like to see what it's like when a signature is invalid.

What's the simplest way to do this, given that it's going to
be a one-off occurence? Perhaps there's some way to
save a message to disk just after it's been signed but
just before it's sent so that I can modify it in a text editor?

thanks,
MC