Hi,
I fixed a rooftop A/C for a friend, and it took me about 4 hours to go home, get my tools, solder on a new freon fitting, fix the leak, and evacuate, recharge the system. At the normal rate of $75 per hour, it would have been a expensive fix. Some places charge more per hour than that.
Normally the replacement A/C will come with a new housing for the inside. Any reason you want to keep the old one? It might fit, it all depends on how much has changed over the years. Normally they keep things about the same, but the replacement might be a bit different. If it is only to keep the "Solar on" light, then I would disconnect that LED from the old inside frame, and put it someplace out of the way inside the roof, leave it there, out of sight. It really does not do much to keep the battery charged anyway. The LED solar on light is on when it is light outside, off at night. Really no help to tell you it is sunny out anyway.
Good luck!
Fred.