What kind of damage can a bad timing belt do in a 98 Plymouth Breeze?

Your pistons are probably fine but if your timing belt broke you can almost bet that you have a couple bent valves. Get your mechanic to do a leak down test first. If he says he can`t do it without a timing belt on it he doesn`t know what hes doing so take it somewhere with skilled techs. This test will tell you if the head has to be removed or if you can just replace the timing belt.