I've been looking. Early Rockhoppers actually came equipped with Deore XT Deerhead groups. The last couple on ebay have gone for several hundred. My local, Cleveland OH, CL has really inflated prices.
I was more interested in what size BB I would need to get a decent chainline. Stock they used a 122mm. I know as a SS it wouldn't need anything that big.
A Rockhopper will have a 68mm English threaded BB shell. As for spindle length, that depends on the crankset you get. You usually get a crankset and buy a BB to match. With a donor bike, you could use the BB and spindle and have it correct. The donor doesn't have to be another Rockhopper.