I didn’t convert the Cairn draugr but came up with the stats that fit Runecairn for my own version.
Some general guidelines I use:
HP -> Resilience Armor -> Defence WIL -> Split between Wits and Spirit, adjusting Wits according to how cunning/crafty they are and Spirit according to how self-possessed/confident/controlled they are Critical damage -> Dire Strike (with judgment) Speed: start with a base of 40 and adjust according to how fast/slow they are
Yochai has some good advice for converting monsters to Cairn from other systems, the principles apply to Runecairn as well, though not in the exact manner.