I looked for a piston for my 1927 Humber 350cc for ages and in the end gave up and looked through the Mahle and KS piston cataloques to find something I could modify to use. I ended up with a Ford Fiesta piston which works fine, but remember to leave smaller piston to bore clearances with modern low expansion alloy pistons.
I am just rebuilding the engine on my 1923 Abingdon King Dick 500cc after advertising in Old Bike Mart and the VMCC mag for 3-months I decided to do the same as I did on the Humber and use a modern piston. The original cast iron piston is cracked and has a lump out of the side. I am using a VW deisel piston in this engine and the weight is very similar to the original piston.
If you do find somebody to cast a new cast iron piston for you, please let me know as it may be useful to me in thefuture.