Isn't the front brake master/caliper on an S440 a Brembo? If so I would definitely stick with that brand, as it's one of the best in the business and should have a specific piston size for the S440 application....
I've switched models over to the MC that comes on KTM dirt bikes which is also a Brembo. I've had some luck with finding the S440 Brembo mc (model num 10.4889) but trying it to see how it performs.