Bruno,
To keep appearances 'correct', how about using a
'hidden' antenna of some sort? You can still have a
'dummy' antenna on the rear fender.
How you 'hide' the antenna depends on what you have
to work with and your ingenuity/imagination, and how
much 'compromise' in appearance you're willing to put
up with. If you think a fibreglass whip instead of a
steel one would look okay, it would be fairly simple.
Just snip the internal wire to the right length. Well,
maybe not so 'simple', but still possible.
Or, how about using very small conductors and glueing
the antenna to a window (burgler alarm tape?)?
'Doc

PS - Put a 2 meter antenna on a '51 Ford p/u like that,
it worked 'okay'...