The overheating BMC 1.5

Everyone seems to love the trusty old BMC 1.5. Even my unofficial Chief Engineer says it will go on for ever. This is my fear. That it will go on poorly long enough that there's not sufficient justification for removing it and fitting something either:

a) much older
or
b) much newer

The lastest saga is that on our recent trips, the engine has overheated rather too quickly, requiring a shut down and then a slow cruise for an hour. Not particularly pleasant in the total darkness.

It turns out that the skin tank is only 4.5 square feet and should be 8 square feet. It is being proposed to double the size by putting an external skin tank.

I am assured this has been done to loads of boats in a similar predicament (Why are there loads of boats in a similar predicament? Was there a whole generation of boat-fitters who couldn't fit an engine in a boat properly?) I can't help feeling, however, that a big tank stuck under the counter on one side is going to affect the swim of the boat. Also, I hate the thought of drilling holes in the side of my boat. Below the water line.

I heard that marinised Isuzu diesels are very cheap right now because Isuzu are withdrawing from the market. Tempting.