Yes, that one really marks the end of an era - after that, they started going cheaper and cheaper chassis.
Bit of a shame the victa has a vertical pull briggs, but it started 2nd pull. Has a bit of a main seal leak - I tipped in a bit of Rislone stop leak to see if it helps. If I have to take the blade carrier off to fix the main seal, a nice 2 stroke will be taking the briggs' place.
The Full Crank Side Pull Mustang that supersedes it used the same chassis at that one by the looks of things, but with a new design dress cover that still screwed to the chassis in the same fashion, along the high arch chute. It lived on until at least well into 83, if not, right until 84 when the PT arrived.
There was also a secondary high arch thumb latch chassis for the FCSP, positioned under the Mustang in the hierarchy called the Corvette and the Corsair models, each in turn, with different levels of features.
That chassis also existed since about 1976 for various Victa and store brand models.
That "second tier" thumb latch alloy high arch chassis ended up taking over for the 1984 Mustang model with the Powertorque engine -which I have in my collection!
Soon after, Victa used the Vortex based thumb latch chassis for their premium priced thumb latch series PT line up.
Regards the mower with the Briggs, I expect you may be able to fit a powertorque two stroke but not a FC as the bolt pattern is smaller size than the Briggs/PT.