Engine Problem- Piston

[Tony, NY]

I have this "forever" project AV8 with a '51 Merc bored .060 over, and numerous other things done to it, that's 10 yrs in the making. It was running and on the road briefly this year. It has probably 5 running hours on it.
Here's what's going on. Last week I started it and revved it up a little and all of a sudden smoke starts coming out of the breather. Pulled the breather and could feel pressure pulses coming out. Shut it off and did a compression test.
Zero on number 4. Pulled the intake, nothing unusual. Pulled the pass head and found a hole in the piston.
Apparently, last winter when I tore it down to do something, it swallowed a small nut down the intake passage when it was off. No damage to valve.
So today I find out that those pistons (Silvolite cast) are no longer being made. What can I do? Put a different one in that cyl? Can I replace them all without doing any machine work? Thanks in advance.