So ..... I'd think if you obtained Bersa springs from EAGLE IMPORTS (their distributor in the US) or maybe Numrich or Brownell's might have them, you'd be in business.
Wolff doesn't make them but getting springs for an 8 or maybe 10 rd Bersa mag (for length) if not right length and then clipping it and bending the floor-plate holder "tail" would work. In an emergency, I'd be afraid the shorter spring might not be strong enough for a full load. Maybe 5+1 would at least give you the original capacity for the P64 in a round-about way... I was hoping Wolff would make the 7 rd spring in a +10% weight that might make up for length.
I put the Bersa spring in and it loaded fairly well with me holding the floor plate on. Never tried to fire it and still would be concerned that the full 6 rds of 9X18 might be too much for the smaller spring. But here's the info FYI!

