There is currently about a handful of Solex 44 PAI carbs listed on Ebay. They should not be hard to get for a reasonable price.
Any of them should work but you may have to move your jets over dependent on where the used carb is coming from.
On re-threading you are probably best off by putting an insert in there.
One brand that comes to mind is HeliCoil but there are many others out there too.
You will have to drill and tap the hole to fit the insert that will make it fit the fuel nut. There are complete kits that come with the drill, tap and insert.
You will probably have to go with a stainless insert to be compatible with the fuel.
Hope this helps.