I’m using UE4 on Linux without issue, even with C++ scripting.
Only the download of content could be painful, and some plugins that are not compatible with Linux
You really don’t have to find a “copy” of windows 10.
Its ISOs can be downloaded for free on microsoft website.
You can use it for free too
After 30 days , you need a key to activate windows.
If not, some customization options will be disabled but you could still use it !!!
If you have an activated windows 7 legal version, you could try to gain a win 10 key by upgrading your system. Sometime your official key would be updated too.
More info here
And legal windows 10 keys can be buy for a few dollars on the web