If you can find a stock tach cluster it should be plug and play, maybe one or two wires to connect. Cluster from a Ranger will work equally well.
I know where there is an 85 B2 near me. I don't know what cluster is in it or if it works, but I'd go pull the cluster for you if you are interested. Just give me price + shipping.
I don't know if you can take a tach head and just put it in the existing cluster but I also know where there is a tach in a broken cluster.