Hello,

is the smoke coming out between the cylinder and cylinder head? When there is smoke inside the engine, I'm afraid that the cylinder head gasket is rotten. This happens usually in cause of a gap on the cylinders top between the gasket and the timing chain case. When the gasket gets damaged there, hot gas will get into the chain case and the whole engine. You may hear a funny sound, while the engine is running.
This gap was a problem for the early TR1s. In the 1980s, Yamaha fixed that, because it was a real disease. Yet, some people were not aware of that service, and missed it. Others thought it was not necessary.

There are two ways of getting rid of this problem: Closing the gap by welding (soudage), or putting a matching piece of steel in the gap, that prevents the gasket from getting blown apart again. You may find more information in the technical part of this website.
When You take off the cylinder head, You will see if there is that gap on top of the cylinder or not. If it is there, what I strongly suppose, You should fix it or have it fixed, because otherwise this may happen again. It even makes sense to do the same with the other cylinder, once You got tzhe engine out of the frame.

I'm sorry that I have no better news for You. Je suis désolé.

But hang on,
Stefan

Sorry i missed a word.
Read " it smell more like burning oil than exaust gaz"

the only other explanation for this phenomenon would be a hole in a piston. But this is very, very unlikely. It's something that happens frequently with two-stroke engines and sometimes with high performance engines. Never heard that it occurred in a TR1.
And serious trouble with the piston rings You would have noticed by enormous oil consumption and very, very poor performance. The afflicted cylinder wouldn't work any longer and the bike would not go faster than 70 km/h.

ST