As previously posted, the number on the lower tang is not a serial number, so that tells us nothing.   

The Model 17 was by far the most common.   

I've never seen one with the "flat spring to keep the lever up", so I'm no help there.  Droopy lever syndrome is endemic to Favorites and Model 44s. As soon as the link holes and pins wear they get loose.   Best of all possible fixes is to ream the holes for 4mm steel dowel pins, which will take up the slack, and will last longer, too.   

Is the mainspring an "arch", or a flat spring screwed to the lower tang?   

Extractor at 7 o'clock or 6 o'clock?   

One definite "plus" to the 44 and the Favorite is how easy it is to get a sweet trigger.   

Bores do tend to be trashed, since these little rifles had to live through the era of corrosive priming.

Rather than a liner, consider replacing the barrel altogether.  I've had excellent results buying take-offs from Marlin Model 60s, setting them back and cutting a tight competition chamber.