I would suggest, unless your gas safe to leave well alone. To fix up the heat exchanger (unless you mean the plate heat exchanger) will no doubt involve removing the heat exchanger/burner cover. This is part of the gas train. What if the seal on the cover is split or damaged, what if the repair goes wrong and you find you need to remove other parts or if the **** really hit the fan and you damaged something else or the repair didnt work. If you are gas safe you will at least be able to leave the appliance safe with the relevant paperwork. What boiler are you looking to repair?