A video that shows how to install a brass OPV and brass-necked pump on a Breville (Sage) Dual Boiler, model BES920XL. The process is probably very similar for model BES900XL, which is the older version of this machine.
Brass-necked pump: https://www.amazon.com/Ulka-EX5-Vibratory-Pump-Dramatically/dp/B00B8KBVZE/
Brass OPV: https://www.espressocare.com/products/item/vibiemme-bypass-valve-opv
Compression fitting: https://www.ebay.com/itm/1-8-Bsp-To-4mm-Compression-Elbows-Fitting-Air-Fluid-Qty2-b231/123782929227