Crank bearings & seals can be had at any bearing house.
eBay seller 'gotfishon' has any gaskets you would ever need, and they are all new, as he produces them.
Carb kits from the big Walbro WR carb will have most of everything you need except for the duck bill.
And duck bills are a standard industrial check valve insert, and available from an industrial rubber company in Dallas TX.
Ask the people on vintage kart forum, and they will have their name & contact info, but you will probably have to buy a bunch of them.
There is a seller on eBay that has done that, and selling them 5 at a time, but the price is a bit steep.
And there is 2 different materials used for duck bills, one for alky, one for gas.
I prefer just to run the big Mac carb like the Walbro HR carb that works just fine without a duck bill for gas.
If you knew the story behind the duck bill, you would see that it was a bandaid for the original bad idea McCulloch had by trying to run without a venturi on their version of the Walbro 'carb', which was more of a pulse injection set up.
Rings? You are on your own on that one.
There are people with stashes of them, but eventually they will run out, and I doubt anyone can be talked into making a run of them again.