Prebuilts never support layout changes, what would be the point you bought a ready to use board, modding it would void the warranty.
Beware, very few boards, prebuilt or custom offer a 7u spacebar layout. Most customs seem to either go with a wide spacebar and no Winkey at all or they go with a standard layout. What you're after just isn't common at all. As for spotting it, the pcb will have 3 sets of holes for the left ALT switch and 2 or 3 for the Winkey.
As to modding a pre-built
If it was just the spacebar it wouldn't be such a big deal, the problem is you need to mod 4 other switches.
You can find the trace path and re-wire them, the plate can be just cut straight across. Where it gets tricky is re-drilling the pcb, if another trace runs through where you need to drill you will need to kill that trace (at both ends) and put in a jumper wire each time. It's doable but you may end up needing nearly a dozen jumper wires. Also keep in mind the plate would be the only thing holding the moved switches. It's quite a lot of work just to shrink 2 keys.
One PCB you can buy that supports it is the old
Phantom PCB, they aren't too difficult to find, you might want to dig through the old thread to be sure (it was a group buy item), then buy a keyboard it fits, there's quite a few, as well as aluminum cases. Beware, there be dragons here... It will fit many keyboards, but to get the 7u the plate will need to be cut, from there you can use a stock case, the Tex case, and the YMDK case, it will NOT fit the Vortex without cutting a hole it in the bottom. There are universal plates you can buy to fit this which support 7u spacebars, however, here too be dragons, there's no easy way to mount them, most used foam I bvelieve, if you have a Tex case, you can 3d print some
adapters. It may bolt into a YMDK, but I can't be sure. If you go with these cases you don't need a sacrificial keyboard, but you will need stabs and a cable.
This option is a lot of research and may be hard to get parts for, but you have the option of doing it easy by throwing money at it or going cheap and doing some work. With used parts you might get it done for $200 if you manage to salvage the switches, if you buy all new or go aluminum case and nice switches so you don't have to desolder you could be in it for nearly $400.