A friend has one of these in 22 LR and wants to know if the action can take the stress if he rechambers it to .22 WMR. Anybody know what the SAAMI spec is for strength of this action?

I know it will take the pressure of the .22 caliber magnum because I own one. Mine has the full octagon barrel on it. I think it also comes in a .17 rimfire, but am not completely sure about that.
Tom Purdom

Not to strong. I would worry about battering it loose. It was not made out of tough steel. They made them in 32 RF. I converted one of them to 32 S/W Long. That worked fine and did not get loose.
As an aside, I once rebarreled a Spencer to a 45 cal cartridge based on a shortened 45-70. It held about 30 grains of 2f. That seemed really wimpy. I tried a very mild smokeless load using unique. It only took a couple of shots to loosen it up. I would not alter your Favorite for a high pressure cartridge. I don't think the steel is up to the task.

Two other factors. The 22 mag bullet is jacketed. The barrel is soft steel. It may cause accelerated wear. Finally, the value of the rifle will be reduced my modifying it.

Scota4570 brings up a good point, I don't know if it's really relivent or not, but are we talking about one of the old ones or current production?? Might make a difference, but not being a gunsmith or well informed I'm just not sure. I guess my first thoughts were of the current production units, but was reminded that may not be the case.