Wow - what a spot to be in. I always enjoy job more when I'm giving my best but I can see how the motivation wouldn't be there. Anyway, can you use your waning time there to both do a good job but learn something new, even if it isn't major? I don't know, maybe just showing up and doing a reasonable job is all that is necessary. I'm sure you've already thought of this but you may/will want to use them as a reference if you don't go with the new firm who purchased your current company so keeping up the 'effort' might be worth it just for that.

Tough one ... best wishes.