There are some modern H-D and Indian flat-top pistons that might prove useful for someone building a modified flathead.
All the pins are too small, but a new pin bushing fixes that.
Bore sizes:
Chief 3.25"
H-D UL 3.3125"
H-D UH 3.422"
H-D XL Evo 1200, FL 1340 (same bore, different compression distances) 3.498"
Corvair 140 3.375"
Corvair 145, 164 (same bore, different compression distances) 3.4375"