as above dont bother with a scrappy one, is it damaged or just blocked? if blocked the first thing to do is remove your matrix and check the outside condition, ie the fins are all ok and there is no major damage and its not leaking

then more than likely its just got blocked up so then pour a load of vinagar into it until the vinagar starts to come out the other inlet, leave over night for it to break down the blockage then run water through it for 5 mins to clean all the broken down crap out, my plumber recommended this to me as this is how he cleans boiler filters