The jpole is long for a straight installation on a tail on 121.5 - that
is correct. However, the 1/4 whip without a ground plane IS NOT 50
ohms. Maybe an an 'L' match?

I did not limit the dipole to 90 degrees!. I said it could be installed
with up to 90 degrees offset in the center. Actually more but there is
some signal cancellation. At less than 90 degrees there is no signal
concellation. A dipole that is bent in the middle is called a Vee
Dipole and exhibits a more omni directional pattern.

The J-Pole can be installed with a bend but that would also change the
impedance, but not excessively. I am only putting out some ideas.
EZ-NEC will model the antenna before you build it.

On a metal plane I would opt for the 1/4 wave whip.
On a composite plane the ground plane is gone so basically the feed
coax becomes the ground. This will work but a matching "L' network may
be required to get the antenna to feed at 50 ohms or so. 1.5 to 1 SWR
or less. The radiation pattern will be distorted.

A dipole can be made that is much smaller than the full length by
adding inductance on each side. It will still perform well and be
around 50 ohms. A dipole that is only 30% the normal half wave will
radiate efficiently. So on 121.5 that would be approximately
(3.7 * .30) total length in feet. The inductance may be added at the
feed point. Each side would be (3.7 * .30)/2.

Short dipoles are quite popular in the ham realm especially at lower
frequencies. Several commercial vendors market shortened dipoles.
Rob NV7F