There are three machinists in our city . . .
I went to the first, who are race-car engine builders etc, and they told me "You have to get a proper spacer, you can't re-purpose something else. We'll have to see what's available . . . ."
The second one is just around the corner from work and I use him all the time for trimming pump impellors, modifying sprocket blanks etc etc - he sorta looked at it and said "It's a piston" I explained what was needed and he's like "well, if you can source a spacer and tell me what the clearances should be . . . ."
The third shop is full of dismantled truck engines and old V8s and resurrected WW2-era tiny flathead English four-cylinder engines. He said "We just need to find the right sized scraper of an old oil-control ring and . . . "
He's the man, I think. I just emailed Rob at oldTimerEngines to see if he has a dead piston with an intact oil-control ring hanging off it . . .
Progress
And thanks for all the advice