Is it possible to revive a pilot who has had a medical condition in a seated position in a small aircraft?
I'm going to preface this by saying I'm not a doctor, however I am trained in first aid and CPR. 

If you are asking whether it would be possible to revive someone who is in respiratory or cardiac arrest to the point where they could fly again then the answer is absolutely not. 

The movies make it seem like you can give CPR and 2 minutes later the person is running around shooting guns, however that's very far from the truth. CPR and rescue breathing are techniques which can, if done properly, keep someone's blood moving enough that radical emergency medicine may save their life. CPR cracks and/or breaks ribs and can cause internal injuries even when it's done right, rescue breathing doesn't generally cause injury, however anyone who has needed either will require days, weeks or even months of intensive care and recovery. 

As to whether it would be possible to administer CPR or rescue breathing to someone in a light airplane then I would say yes, with extreme difficulty and a low chance of success. CPR requires a lot of force, you use the full weight of your upper body concentrated on one single point on the person's chest, unless you could lay the person flat on a hard surface you'd probably be wasting your time, so if the seats reclined a person in the back seat may be able to lean over and do it, although that's extremely chancy of working due to the angle and the cushioning of the seat. Rescue breathing is possible with a seated person, provided you can open the airway, getting the angle on that would be very tricky in the constrained area of a small cockpit. Again, if the seat could recline a person in the back would have a better chance of doing this. 

Most importantly to remember is that if the sole pilot dies or becomes incapacitated someone else has to fly the airplane. Struggling to get into position to perform CPR or mouth to mouth on a seated person would be a great way to snag the stick and send the airplane out of control. If there's a back seater by all means let them give it a shot, but your best bet at survival is getting control of the airplane and calling ATC for someone to talk you through things, it's happened before and has a remarkably good survival rate.