The best alternative to BartPE is SharpBoot, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to BartPE and three of them are so hopefully you can find a suitable replacement. Other interesting alternatives to BartPE are FlashBoot and GNOME MultiWriter.
SharpBoot is a small utility for creating multiboot ISO or USB. You just need to drag-n-drop some iso files inside, and it automatically adds description and category (for supported ISO files, not shown in the screenshot). It uses Syslinux for the menu and bootloader.
FlashBoot is a tool to create an installable clone of Windows 7/8.x/10 (including all your apps and data) on the USB thumbdrive/HDD and instantly transfer your entire OS to another computer, even with dissimilar hardware:
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