Modern mowers nearly always have governors - in fact they've had them since the early 1960s. With your swapped carburetor you have no governor, and if you open the throttle with no load it is only the poor breathing of the Victa engine that keeps it from throwing its connecting rod within a couple of seconds. Back in the 1950s and before, mower engines tended to have even worse breathing than modern Victas, and weren't all that likely to scatter hot metal across the landscape if you opened the throttle without a load on the engine. Most 4 stroke mower engines used to be deliberately made with weak valve springs so they would valve-bounce instead of scattering, and of course they had a governor as well. 2 stroke engines often have the governor internal to the carburetor, which should make them less likely than 4 strokes to have the governor stop working - but having no valve springs, there is nothing else to keep them from over-revving and self-destructing.
You can use your Victa without a governor but it is likely that sooner or later (probably sooner) you'll over-rev it and break the connecting rod.
Apropos of nothing, when I was young there was a practice of having 'engine blowing-up parties'. An unwanted car engine would be set up on a stand, and run flat-out with no load until it blew. Meanwhile the lads would watch it and drink beer (mostly the latter). Of course being 4 stroke car engines, in those days they tended to just valve-bounce very noisily at around 4,500 rpm for a very long time. When necessary the sump plug was removed to hurry things along, but even then they usually just seized up rather than blowing up. Don't try this with a modern car engine - they don't valve-bounce any more, they rely on the speed limiter in the engine computer. Any silliness could result in a face-ful of fast-moving hot metal.
