This is a 16-valve 1.9 liter pocket rocket that I bought new and have driven for 29 years.  About a couple of hundred miles and two months ago, the engine began missing at idle and low rpm, with occasional clouds of pale smoke.  When the rpm got up it would smooth out and run fine, but at a stop light, when the rpm dropped, back came the problem.  It seems to be a head gasket leaking coolant into one of the combustion chambers.  I added Aluma-Seal, but nothing changed.  Today, I added some Fiber-lock, carefully following the instructions and after a test drive, it now misses continuously during either low rpm or high rpm.  I am at about 307,000 miles on the original engine, transmission and clutch.  I always expected the clutch would be the first thing to go.

Is there any hope of a fix for this without taking the engine apart?