These are not too difficult to repair. I've done a fair amount of messing with these traditional Geeps, swapping shells, etc. (being relatively interchangeable, they're really a lot of fun to mess with, putting MPC era shells on TMCC chassis, using the strobe light to make a MARS light in front, and other kinds of stuff).
The absolute simplest way is to do what Chuck Sartor suggested above, using a screw though the front. Although when this ACL Geep was made Lionel had begun using these plastic posts, as Chuck noted the older design just had a single screw through the front and into the hole in that upright metal tab in front that the shell fits over. Lionel probably did the change to hide the screw end, and also because occasionally the shell plastic around the screw hole would develop a crack.
At the time this ACL Geep was made, Lionel hadn't changed the metal frame at all, so the metal tab with the screw hole in it was still present. So simply put the shell down in place on the frame, and drill a hole through the plastic, lining up with the hole in the metal tab. You'll have to measure and judge this just a little, because the tab hole is inside the shell and not visible when the shell is in place. You have some leeway, though, because the head of the screw will cover the hole if you have to make it a little bigger to hit the metal tab hole. After you've drilled the hole, insert a screw, and you're done. Don't over-tighten the screw, by the way. The small screw head will be visible on the outside of the shell, but it's no big deal, and you can paint it if you want to blend it in more, although it's not very visible.
Another option is to grind the broken posts down flush, and then glue small blocks of wood to the shell over the spots where the posts were. Then, holding the shell in place, turn the engine over and drill small pilot holes through the frame holes and into the wood (don't drive it too deep). Then insert screws, and you're done.
Other methods discussed by posters above will work also.
Another modification you might consider is one I always do with these engines, and that is to move the front light bulb forward until it's just behind the lens. Lionel placed these bulbs back so they would also illuminate the cab, but the result is that the front headlight is rather dull. When you reverse, you'll note how much brighter the rear light is than the front, because it's mounted next to the rear lens.
So move the foil (keeps the bulb from melting the plastic shell) and bulb holder forward until they're right behind (not touching) the front lens, and secure them there. If the wires aren't long enough to move the bulb forward, slice in a little more wire. The light in the cab won't be as bright (you'll get just a little glow), but prototypically there's little if any light glowing out of cabs at night anyway, just like your car.
