Trying to find a chemistry to remove a coating

In 1986, Vancouver BC Canada hosted a World's Fair called Expo 86.  Transportation was the theme.  They made a fake highway "Highway 86" with all sorts of vehicles in static display - planes, a 59 rag-top Caddy, a Studebaker, a camper truck, some bicycles and motorcycles - all were covered with what appears to be latex and then cement over that.
 

 I rescued a Honda motorcycle last year that had been part of that display.  The layer under the cement appears to be an off-white material I can scrape with my fingernail.  I can remove most of the cement with a powerwasher, but the layer below is very tough.  I have tried PineSol, brush-on paint remover, DOT3 brake fluid and iso-propyl alchohol.  Nothing has done more than slightly soften the white stuff.
 

 I did manage to chip a bit of the stuff off chromed things like the fenders and they are surprisingly well preserved.  So I want to find something that will soften this stuff with as little damage as possible.  I can email pictures if you give me your email address.
 

 Thanks in advance.
 Brian Barnes