If wanting native on Linux, have to go with Caldera (but be aware which distros that they officially support).
If dead set on trying to run Flexi on Linux, your best bet is to try to see how it handles via VM, but make sure your computer has enough resources to essentially be running two OSs at the same time. But will still be reliant on Windows in the end. Now the nice thing about Linux as a host is that there are plenty of DEs to get the host requirement down to allow for more resources to go to the guest. Depending on what connections that Flexi needs to talk with everything, that could be a bummer going through a VM (it may work, just take some time to set everything up).
Now, if don't want to run a VM, the next best option is to use WINE (compatibility layer, translates Windows sys calls to POSIX ones). However, there is a big caveat with this, it tends to do well with older versions of software (not all the time, depends on how motivated people are and have to work on a particular software) and certainly not niche ones (which Flexi is). Because this isn't using a hypervisor or an emulator, the performance hit is almost nonexistent. The reason I don't have this labeled at the top is because it may not have workable compatibility with such a niche software such as Flexi. But it's performances would still be better compared to straight emulation.
Emulation would be the last option (some will say vm/emulation interchangeably and they are not, there are significant differences between the two and those also translate into performances hits (or not)). I wouldn't even go this route. I mention it, because it may be possible to get a working solution still, but performance will be a drag.