How to rescue the crack on the transparent piece?

If the lens is still in one piece with a crack in it, first thing I would do is dip it in Future.  It should make the crack less noticable, though it may not entirely eliminate it.

Worst case, press lens into a smooth surfaced block of modeling clay, detail side down.  This will create a one time mold.  Fill the mold with epoxy glue- the most transparent you can get.  Let it thoroughly harden- overnite- then peel clay off.

Can you tell me where to buy Future?

Future (Pledge Floor Care) should be at your local supermarket/box store.  Is a floor care product.

Can I apply future on decal?

on a decal that has already been applied to the model in final position and is cured and dry, Yes.

Kelvinator

I cracked the transparent head light piece by accident and wonder if there is a way to undo my error besides buying another whole new kit. 

 

If you have any Micro Kristal Klear, try to "glue fill" the crack from the inside. This is a similar technique described by Don above and may reduce the visibility of the crack.

With TS-13 notoriously hard to use, can Future be the next best substitution for achieving the mirror shine gloss effect on the car body?

Future will work to create a gloss, but so will a lot of other products.  Testors glosscoat is the go-to for many of us.  Urethane clear varnish can be thinned to airbrush and will give good results.  If you want clear acrylic, hardware stores have cans of it that can be thinned and airbrushed, or you can buy spray cans of gloss acrylic coat.  Michaels and Hobby Lobby, and similar craft stores carry the spray acrylic gloss too.

Kelvinator

With TS-13 notoriously hard to use, can Future be the next best substitution for achieving the mirror shine gloss effect on the car body?

 

"on a decal that has already been applied to the model in final position and is cured and dry, Yes."

I use Future to apply decals, float them into position with a brush wetted with future for zero silvering.