The replacement diodes would have to be exactly the same type number as the shared/paralleled diodes. If they were not obtainable, you would have to replace all the diodes in the string, not just the failed ones. If a manufacturer was doing this, I suppose they would build each rectifier using diodes from the same batch or date code.
I would probably try to select diodes with the same forward resistance - e.g. by putting them in a circuit with 30v supply, 5 or 10 Amp load, and making sure that the voltage across all the forward-biased diodes [ 400 to 700 mV ] was as close as possible. However, you could only do this if you had a box of a couple of dozen diodes to choose from.
However, according to this thread:
https://www.boatdesign.net/threads/replacment-rectifier-for-mig-welder.39748/
you can build a 250 Amp rectifier for a MIG welder just by paralleling-up a bunch of 600V bridge rectifier blocks.
Those metal-cased 4-diode blocks can be bought very cheaply:
https://www.ebay.co.uk/itm/KBPC5006-Bridge-Rectifier-Diode-50A-600V/263653215947
Apparently as long as they are all at the same temperature, these blocks will share the current equally ( the resistance of a forward-biased diode increases with any increase in temperature, so the silicon junction self-protects.)
However, you have to bolt them to a heatsink and string them together. I would still prefer 4 broody gurt fat 200A diodes with 3/4inch UNF studs....
