Can I fix this?

[Beneteau381]

As the heat exchanger has freshwater/antifreeze in it might not be a problem.

It might not BUT it is in a salty atmosphere. Also the heat exchanger is a big heat sink and I doubt that a diy propane gun could get enough heat in to the casting to enable zinc soldering.
Bite the bullet, get the heat exchanger off and do the job properly

 

[rogerthebodger]

As above get it auinium welded with a proper MIG Argon welding and it will last forever

 

[Beneteau381]

As above get it auinium welded with a proper MIG Argon welding and it will last forever

TIG argon?

 

[rogerthebodger]

TIG argon?

Yes If you have the skill and aluminum filler rod.

I generally use TIG argon for stainless steel melting the edges together.

If I need to fill the joint, I tend to use MIG, so the filler rod is fed into the joint making welding easer. I have both 316 spool and an aluminum spool and gun.

Different welders have their favored technique

 

[galeus]

Thanks all for your trip time and advice; It's got to be done and will tell you the outcome in due course.

 

[MontyMariner]

The glue/epoxy suggestions all have their merits and I think faced with no alternatives would probably work

A couple of things in its favour - cheap and non destructive.
You can also add can be done in situ and if it doesn't work the other options are still open.

 

[snowbird30ds]

Measure the OD of the filler stub where it enters the header tank, if it will take the 1 1/4" bsp thread talk to bowman about a new neck and tap it out to suit, could be done in situ, a hacksaw blade down the inside of the stub and it will likely come out out easily by colapsing into itself.
It needs fixing so may as well try easiest way first, any welding will need to be pristine clean first and it will have corrosion in the leak, same with just filling with epoxy.

 

[NormanS]

Some years ago I had the same problem. I was able to buy a new fitting with a simple two hole flange. I chopped off the old leaking one, with a dolly grinder, and ground a flat face on the heat exchanger. Drilled and tapped holes for studs. Job complete.