Tiger, you're far from the first to observe this issue with Victa PowerTorque steel decks.
The ODK consensus is pretty much that once they have rusted out around that left handle mount, near the left rear wheel, they are too far gone to be salvageable. The metal there is quite a complex shape, which makes it very difficult and time-consuming to form/fabricate a patch.
That area traps clippings, and as few owners now hose out the underside after use [as even Victa still recommends], it's the first part to rust through. However, by the time that occurs, there is almost always substantial loss of metal thickness elsewhere in the deck - which also needs repair, before the mower can be considered to be in 'saleable condition'.
So attempting repairs quickly becomes a waste of effort. 'Rusty air' [corrosion-thinned sheet metal] doesn't weld at all well, especially with 'stick' [MMA] welding. Then there's the time and materials to prep and repaint afterwards...
All mowers get to the point of being 'beyond economic repair', but this type of light gauge pressed steel deck reaches it sooner rather than later. Just part of the 'throwaway' design era.
In contrast, the steel decks used on 1970's Victas [only on the Utility models though] were heavier gauge and didn't have 'clipping trap' crannies, so many are still serviceable.
As slashnburn says, oxy-LPG is quite good for brazing and cutting [flame temperature is too low for fusion welding of steel], and much cheaper to run than oxy-acetylene.
A dedicated LPG welding reg, which screws straight into a POL valve [as on 9kg LPG bottles] runs under $70 from BOC via FleaBay.
