The first gen horn material is some sort of plastic that's very porous, brittle and as already stated, very sensitive to heat. Standard epoxy glues work fine to bond cracks and does not dissolve this base material if you can manage to get it into the cracks. Thin CA or cyanoacrylate glue works wonderfully in the hairline cracks where epoxy cannot be forced in. These glues are easily available at any hobby shop or woodworking supply store. Besides the glues, standard body shop methods and materials work well to refinish the surface of these horns. An experienced body shop man would make the job look too easy.
But, if you go through all the work and expense to repair the cracks and resurface the first generation horns, you still have a defective horn that's susceptible to heat warping because of the poor choice in plastic used in the original manufacturing process. Not to say repairing isn't an option. If you have the time, tools, know-how, experience, materials and willingness, then by all means go ahead and repair them. As long as you're at it, the entire outside of the horn could be covered in laminated fiberglass resin and cloth to prevent further warping. It's sorta like the DIY'er that devoted many months to restore a 1970's Chrysler. After all the work, time and expense - all he has is still is an old Chrysler that was never worthy of all the time and expense put into the restoration job. People also have different standards of aesthetics. I personally can't have a set of warped K-402's in my listening room, the rough demeanor would bother me. Heck, these horns in brand new condition generally rub the wife the wrong way just because of the outlandish proportions. Add a horn surface that's all warped out of shape and I'm pretty sure most married men don't have spouses that are that understanding. I wouldn't mind a set in the workshop, except it gets much too warm in there. Another option is to repair the cracks with epoxy and build a nice plywood cabinet with a grill and forget about it.
There's a third option! Did you know that a sales rep of Klipsch Inc. will sell you a set of brand new replacement horns for a very reasonable cost if your horns are really 1st generation? The sales are Roy Delgado approved, he took full responsibility for the defective 1st generation horns that I purchased (at top dollar) from an unscrupulous long time forum member. So I would imagine that you too can purchase just the horns, no drivers or stands at a considerable discount.