Assuming the 6.1.2 ISO is the right image, you can burn it to DVD and boot off of it from the new server. From there, it can reformat the drives and install a fresh copy of 6.1.2 on the server. Keep in mind that in order for a sub to join a cluster, it must match the cluster version exactly. Fortunately during the installation from the DVD, you can add a patch to bring it up to the exact version. So if the ISO is of 5.1.2.1000 and your cluster is running 5.1.2.2000, you can point the installer to the .2000 installer (from a FTP/SFTP network location) to upgrade it before it attempts to join the cluster. However, if your ISO file is of a slightly higher version of the cluster, it won't work and you'll have to upgrade the cluster first.
I leaned this the hard way. I was running a ES patch that I unfortunately deleted a few months ago. I then had a sub die and had to be rebuilt. Unfortunately I didn't have the ES patch to add onto the installer and wasn't able to get the server to match the cluster (TAC wouldn't provide a copy of that exact ES patch either). So I ended up having to upgrade the whole cluster to a slightly higher version and use the same upgrade patch on the sub during reinstallation to get it to match.