It may well be possible to get it to work well enough again. Obviously it having already sooted up would make me and probably most other fitters look very very carefully at it. I'd really want to be 100% sure that the problem would not return in the next couple of years.
It is a simple unit being essentially no different from any of a dozen models of it's era.
Essentially all that's needed is a thorough clean out. Check the flue after a 5 minutes operation that it draws well enough to clear a smoke pellet and check that all the flue gases are going upwards when in operation.
However there is no margin for error on this job. The cause of why it sooted up must be found and corrected. This is a plausibly a matter of life and death.