Hi,
Welcome aboard.
The first thing you need to do is use the search function with "P51" in the search box, as there are a few threads on this gearbox, including the rebuild manual.
Second thing is, there is no replacement for these bands. Toyota don't have them anymore and nor does Porsche (the P51 is a ZF/Porsche patent gearbox).
The only solution is to try and get a new one custom made - assuming you know the dimensions it needs to be made to.
Have you got pics of the worn band?
seeyuzz
river
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