How can I fix an apollo 625 dvi?

I have an Apollo 625 DVI wall mounted direct vent gas fired boiler manufactured by Thorn EMI Heating, which has a crack in one of the water pipes through the heat exchanger. The crack is on the exterior of the exchanger at a bend. Is there any way to repair this crack? If not, how do I find a replacement heat exchanger since I am led to believe that Thorn EMI Heating is no longer in business?