This is easily rectified by getting a non-legion X5 slide, which are available stripped from Osage County Guns (probably other places too, but I got mine from Osage). Legion X5 with a non-legion slide is a fantastic package for not a ton of money.
At this point though, I'd probably just buy a custom shop FCU and source the various parts I want individually, but that's me (and I like tinkering)