sm 420 info:

http://kinzers.com/don/landcruiser/transmissions/index.html

also has info about the sm 465 as well. (for future reference, if anyone wanders in here)

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novak was mentioned above,

https://www.advanceadapters.com

has various things listed as well, dunno if they have what you need tho'. worth a look.

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duckduckgo.com

transmission parts sm-420

looks like lots of pieces are still available.

If you can't find it online, stop by a local transmission shop and ask them.