I would be very surprised if there isn't a map of 19th-century town roads and/or property lines in Alna available somewhere. A quick online search of the collections of the Maine Historical Society shows they have several
18th-century maps of Alna, for example this "Plan of Lots on the Sheepscot River, Alna, 1798" [
http://www.mainememory.net/artifact/12943/enlarge] -- right place, just a hundred years too early!
With regard to stone walls, just because there's a wall there doesn't exclude the possibility of an even earlier road. When I was in graduate school I spent some time researching the history of a large tract of forest land (much of it once farmed) that's now owned by Yale University in the northeast corner of Connecticut (partly in Woodstock, CT in fact!), and so spent a lot of time in the town land records searching for deeds and following the chain of ownership back. I found several examples of old town roads that had been discontinued in the 1800s and had just disappeared from the landscape as adjacent landowners (to whom the roads reverted) just moved the walls over or threw up new walls and expanded their fields to occupy the new ground.
It sounds like a trip to Alna town hall might be in order. I would love to take part in this research but I don't know when I will be back up again this summer -- maybe not until August.