You can swap to the RancherOS image which has the slug rancheros and the username to connect is rancher instead of core.
eg. so your Machine options should look like
MachineOptions = ["digitalocean-image=rancheros", "digitalocean-ssh-user=rancher", "digitalocean-access-token=XXX", "digitalocean-region=sgp1", "digitalocean-size=c-2"]