I have some ideas, but I think you'll get more, and probably better ones if you ask at www.johnbridge.com. I would not want to try to attach the cbu with tapcons. Drilling the holes and then trying to get them to seat in the cbu risks having the leftover plaster let loose.
Chipping the plaster off with the right tool, probably would go pretty quickly. In theory, you could tile directly over the cinderblock. I think you might be able to set Wedi panels directly on what is left, if it has a fair amount of surface area. These panels are fairly expensive, but would also add what may be some very much needed insulation (they are special foam panels with a coating of a special cement product). You can tile to them directly, and you might be able to just thinset them to the existing bits of plaster. Not sure, but it might be worth looking into. www.wedi.de is the parent company. They have a US branch, but I don't remember their website - you can get to it from the one above, though.