I had a boat with a BMC 1.5, lovely small engine, but they do need looking after and many were raw water cooled, lots have have 'bodged' repairs and all have their own 'temperamental' way of working. Most now require a good overhaul and once this is done, will if looked after carefully run for years.
Mine was raw water cooled originally which caused lots of problems, changed this to indirect cooling with a heat exchanger, cleaned all the water cooling channels on the engine, over hauled the cylinder head, new valves, guides, it ran like a dream.
They really are almost reaching classic status, spares can be tricky because there were so many variants, but overall there is not much to go wrong.
The other little problem mine had was never fill the oil more than half way on the dipstick or it would throw it out of the rear crankshaft oil seal.......
Oh! and so many people feel nuts on them should tightened with molegrips, I had a good collection of AF spanners from fiddling with BMC cars.
Now my boat is Gardner powered............it's too late to say 'long live' British engineering, it was killed of years ago by a lack of investment.