Glad to know you got a carb. I suspect it leaked a bit around bowl to body joint so owner thought tighten it more, hence the damage. There should be a gasket around top of float chamber.
The jet might be saved with a bit of careful ‘adjustment’ with pliers. The important part is the hole in the jet, not sure how critical the damaged part is, so long as it does not block the hole. I guess it could be there to ensure fuel is picked up when level drops in the float chamber.

Found this web site for a company making Amal carbs based in Salisbury, they also do SU and Zenith carbs.

https://amalcarb.co.uk/?___store=amal

If all else fails , I can send you a good jet for a pattern to have one made (sorry I would need it back) , the carb it’s off is a spare one for a Rowtrac that I’m now working on , which strangley enough , although a Villiers engine it has the same carb as a 56A.
I would be fairly confident that the jet sizes would be the same.

Andy.

It could be worthwhile contacting the chap at Saving Old Seagulls as I have seen from his spares pages that some models of Seagull outboards use a twin jet Amal carb very similar to the one that you are seeking jets for, he is also suggesting that brass radiator bleed keys can be used to remove the jets with modification.
You would need to check the jet dimension as I very much doubt that he will have a parts cross reference from Seagull to Simar!

https://www.saving-old-seagulls.co.uk/index.html

I can confirm that the key for the Amal twinjet carb on a Seagull will fit the jets in the Simar Carb.

Andy.

Reply from company in Salisbury, no spares for that type of carb.