A piston is not that hard to source, I think your engine rebuilder had a slack attack.
Get all the dimensions, including gudgeon height, and try the following.
Mick Cashell Engines & Parts - Located in ASCOT (QLD): Car Accessories - Retail - AussieWeb Local Search
Australia's largest wholesalers of Engine Parts
https://www.ebay.com.au/sch/i.html?_...saApzYEALw_wcB