To just start snapd you could indeed, but there is a lot more systemd technology used…
Snaps are readonly squashfs filesystem images, when they get installed a systemd mount unit is generated so that the filesystem image gets loop mounted under /snap in your filesystem…
If a snap contains services, systemd daemon units are generated at install time to manage these services…
You would have to patch snapd and write code that does all the above in a sysv-init compliant way to actually replace the systemd requirement…