Wow, yes, your unit has been 'somewhat' repaired. A little bit surprising as the output stage is pretty robust and it has the engineering change to add output clamping (the two large stud diodes with red and blue wires on the aluminium bar).
I don't think minor modification of the connector is going to to make any difference to frequency performance.
I'm more concerned by the length and thickness of the wires going to the output transistors. I think they are long and inductive enough to affect the waveform at the 5MHz end of the frequency range, and probably cause crosstalk with other components on the PCB. The thickness is an issue for stress on the transistor legs (you mentioned that you broke one at the start).
If it was mine, then I think I would try to use thinner short leads as pictured in my original state unit (the missing sockets aren't an issue as there are only 3 wires to solder on each transistor). You could probably even get away without heatshrink with short leads.
I would also try to return that ground tag to a solid chassis grounding point with star washer, rather than sandwiched between two anodized aluminium surfaces with heatsink compound.
Good to see that the Rifa X and Y caps are already missing, actually it looks as if they were never fitted on your unit.
EDIT: Photo of the rear of the BNC connectors added, does yours have the 8R2 resistor on the main output connector? Maybe it is missing from your unit, causing the connection problem?