You can pull it apart, measure the powder charge, and determine the powder type. There are only a few types of powders that will produce the advertised velocities at the measured charge weight. Cross reference this against a load manual, and you can find a similar powder. I'd bet it's close to W748 or W760. It may even be WC735 military powder (get it from higginsons. Or use H335) Then find a bullet as close to the one used as possible. FMJ, length, cannelure, etc. Use a Lee Factory crimp die if the factory ammo is crimped, and seat to the same OAL. Use remington cases and primers.
That's as close to factory as you're going to get.