Why limit yourself to Mojave and miss out on newer features, as well as security features? Buy Parallels or VMWare Fusion, create a macOS 10.14 virtual machine, install Aperture in there (and any other 32-bit software you use), and you're done. Upgrade your primary operating system when you're ready, and keep using your 32-bit programs in the virtual machine. Virtualbox represents a free alternative to Parallels and VMWare, but its performance may not be as good.
The virtual machine will add some overhead and provide a small to moderate performance hit, depending on what you're doing and what your underlying hardware consists of. For Aperture, though, performance should be just fine. You'll also lose a bit of time, as you're essentially opening an application to open an application (which can feel silly), but it'll get the job done.
This is a great option for someone who wants a bit more time and already owns virtualization software, or who has limited needs and wouldn't be impacted by a performance penalty (as you might experience with VirtualBox). If you're looking into buying virtualization software just for Aperture, though... don't waste your money. Just bite the bullet and get Capture One or an equivalent. Capture One Pro didn't fully replace all of Aperture's features for me, and I need to still use Photos for things like searching for photos by geotag or facial recognition. As far as editing goes, though, Capture One is very far ahead of Aperture at this point. Particularly if you're a RAW shooter, do yourself a favor and unlock more of your camera's potential by going with a modern program.