I am going to assume that the cracks are in the origional plaster ceiling..........not gyp board. If the cracks are just "surface" and there is not separation from the "keys" that hold the plaster to the lathe, you are probably in good shape in that the existing ceiling isn't ready to fall down on the coffey table. I am going to let others make the final suggestion, but even if you have to "scrape" all the gunk off, you can apply a skim coat over all the scrapes and refinish a perfectly good ceiling. You can also apply a 1/4" gyp board overlay.
However, if you plaster is ready to drop, there are mechanical fasteners available now that will allow you to "recynch" your existing plaster. I have a friend who is ready to attempt this proceedure on a old house. You can then follow the above proceedures.If it were me, considering that the plaster ceiling will come down easily, I would remove it and replace it with a gyp boad ceiling or gyp lathe and plaster ceiling. Won't take long, and the job is done right. You can rewire that ceiling fixture while you are at it.