I have a 400MB bootable Bart's PE partition on an old netbook, "I've sense lost the cd". The other partition on that drive is XP. I'd like to connect the drive to my usb and use the Bart's partition to create a bootable iso, "some of those tools I cant even buy anymore". My current system is a 2008 imac with Ubuntu 20. Is this something I can even do?

Thank You for any help you can provide.

Since this is Windows based.
In Ubuntu I would make a dd image of the Bart partition with no byte size specified since it's so small. Assuming it's attached to your computer via an adapter and it is /dev/sdb first partition:
You'll have a 400MB image file in your home. Download VMWare Player and install it in Ubuntu, create a Windows XP virtual machine with a drive of say 450MB. Boot Ubuntu live in this virtual machine and have the BartPE.dd image on a USB key, create a primary partition with Gnome disks and mark it bootable, format not necessary as it's part of the image but don't hurt. Apply the BartPE.dd image to the virtual machine's partition with dd. Download Hiren's BootCD PE (click on ---> HBCD_PE_x64.iso in "Filename" field at bottom of page) to help make it bootable if it's not able to boot.

I used to use nLite to create custom WinXP iso, I have an XP Pro SP3 (activated) ISO, and a copy of nLite-1.4.9.3.setup.exe.
For later versions of Windows I use tools from these guys. They have tools for Win7 and up.