Repairing a copper heat exchanger is very difficult, by way of brazing, believe me I've tried it. Getting enough heat into it is a problem as is soldering. I've tried it on two. One split around the brass nuts, in the end I had to buy a new one.
The second time, the exchanger around the exhaust split due to ice, leaving a two inch long crack. I couldn't afford to buy a new one,so I did the following;
Firstly make a sleeve from stainless steel sheet plate, roll it around the exchanger to get the right shape, smother the area and sleeve using "wet grab" and then clamp the sleeve around the exchanger and leave to dry solid. The either end of the sleeve use a jubilee clip, just for extra strength.
You can remove any mess or excess, using WD40. I've had no problems with it holding since.