I'm thinking of an upgrade around 1970, in parallel to the Bundeswehr's real world Leo A1 upgrade program. This would be a decade before the Leo 2s came onto the scene. The upgrade could also improve things such as night vision.
Yeah, I take it that's the Belgian Jagdpanzerkanone JPK with the tracks, transmission & final drive from the Marder along with low light intensification, a laser rangefinder and SABCA fire control. However because they were coming out as T-64 and T-72 were being deployed the L7 didn't have the oomph to penetrate them frontally (although neither did the Rh-120 as it turned out) so they went to missiles instead.
The L7 Low Recoil gun would fit but talking to a JPK driver he said there'd be issues fitting in the ammunition. However I think he was being a stickler for mounting it on the firewall facing the breech.