Well I figured out a way to repair them based on some advice received elsewhere.
I wrapped a couple of rubber bands around the cracked section to put tension in it.
Then I dropped them in a cup of very hot water for a few minutes to soften up the rubber.
When they softened up the rubber bands squeezed the crack closed,
I took them out and let them cool down.
When they were cooled down I took the rubber bands off, pried the crack open a little and let some superglue "wick" into the crack and then put some clamps on them to hold the crack closed while the glue set.
I noticed at that point the they had warped a bit and wouldn't lay flat anymore.
So after letting the glue set up a couple of days I dropped them in a cup of hot water again to soften them back up and clamped them down on a flat surface until they were cooled.
VOILA' nice and flat and no more crack.
I then used the edge of a sharp knife to scrape away the excess glue and shave down the slight uneveness at the crack.
They don't look perfect, but good enough for the condition of the rest of the gun.
I'll probably end up using something to fill in the remaining surface crack, maybe do a little sanding, and then recutting the checkering lines to improve the appearance even further.