Hi Setzy, welcome to Outdoorking.
Please post the type identification of the JAP engine, so all this will make sense in the archives.
Stripped threads in old carburetors are a recurrent problem, and the first issue is whether there is enough meat around the old thread to allow you to fit a helicoil and repair the thread that way. Even if there is enough meat for a fine thread, you then need a helicoil for what is probably a BA or maybe BSF thread, and that won't be easy to get. If you use a modern thread, you will have to make a new mating part, and there will be some loss of originality. So, it is usually more practical to replace the carburetor unless it is a really rare type.
It seems fairly unlikely you can get a part for your cone clutch, but there are a couple of British sites that have some surprising things. When Deejay's internet connection (in Wagga Wagga) starts working again he may be able to advise you on that. Meanwhile I suggest you post pictures of the surviving parts so we can have some idea how much trouble you are in.
It is possible to weld most cast iron, at least grey cast iron, by using special rods in a stick welder. If you post a picture of the damaged roller, and a way to understand the scale, we may be able to assess whether it is more practical to weld up the hole, or alternatively it may be necessary to weld in a patch.
