Ex Mac user of 17 years (now Linux) and keyboard enthusiast here.
The crucial difference between Mac and Windows layouts is not whether there's a Fn key to the left of Space, but the order of the Option and Command keys.
Mac has Command next to Space, while Windows has Alt (= Option).
If you're anything like me, then your muscle memory (after a couple days or weeks of accommodation) won't give a flying fuck of whether the Fn key is missing. In the long run, it will adapt as you switch back and forth between keyboards.
Your muscle memory, however, may punish you hard for reordering Option and Command. So focus on fixing that. Most newer keyboards have a firmware setting or physical switch that rearrange the scan codes.
If yours has no such setting, MacOS has a per-keyboard software setting that allows you to remap those two keys specifically, but I'd advise against using that because it never worked reliably for me. Instead, email the vendor and ask for advice.
Once you got the scan codes reversed, consider switching the keycaps accordingly to make labels match (if they're the same size. Otherwise, ignore the labels.)
YMMV but maybe it helps.
