Hi Nate, Hope you are well.
I rebuilt some Mangusta rear calipers last year and was able to buy all the parts off the shelf from John Freeth at performance braking here in the UK. John worked as an engineer at Girling for 25 years so knows a thing or two about these brakes!
His replacement pistons are nickel plated mild steel as original. If you have access to stainless ones for similar money that would be a better choice. John can sell you just the seals if required.
http://www.performancebraking.com/
Johnny
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