You can try the following (make sure that you have a backup of the original virtual machine before you try any of this).
This assumes that your virtual machine's guest operating system is supported by Fusion 11.5.5.
Create a new custom virtual machine with Fusion 11.5.5 on Mojave
After specifying the virtual machine type, you;; be asked for the virtual disks. Opt to use an existing virtual disk, and navigate to the VMDK file of your original virtual machine located in its "bundle" folder. Choose the file with the .vmdk extension that does not contain anything extra such as -s001.vmdk .Select it and opt to make a copy of the virtual disk.
At the final dialog, choose to customize the configuration. Make any changes needed to make the new VM match the original.
See if the VM will power on.