By sag you mean you can push the lever all the way up but it does not stay there and drops down a bit, look at the screw in front of the lever and make sure it didn't just loosen up, if it's tight then the little spring that rides on the lever might have lost it's tension or broke. Not sure if they can be bent back into shape to apply pressure on the lever or if they have to be replaced once they loose their tension.
I had the same problem on my 39Carbine, not only did it sag but it wobbled about a quarter of an inch either way. I visual checked the spring and it didn't appear to be broken. I was making up an order at Midway and as an after thought had them throw in a new lever spring as well as a new screw that holds it in place ( tiny screws always jump out of my hands as i'm trying to fit them and get lost in the carpets). Took about 15 minutes to replace it after which the lever snugged up tight and the side to side play disappeared, hope this helps.... Don't know the Midway part numbers but type Marlin 39A on their search on top of the page and it brings up about five pages of 39A goodies. Dan