You have several options, I think.
1. See if you can make your straplocks work with the original screws.
or
2. Fill the original screw holes with plastic wood and re-drill them for your strap lock screw size.
or
3. Use the old carpenter's trick for sizing down wood screw holes - drop some carpenter's glue in the hole, drop in a couple of wood toothpicks trimmed to the same depth as the hole, install the new, slightly smaller screw, allow to dry overnight before putting any load on it. (Yes, it works.)
or
4. See if another brand of straplock has the same screw size as your originals
or
5. Forget the straplock and install a big stainless steel washer between the original strap holder and the body. (Not elegant, but it works too.)