The Triac is the final drive to the Motor - all components on the PCB do something useful else they wouldn't bother including them. A new triac will cost about £2 perhaps and will need heat sink compound between it and the heat sink for thermal conductivity.
You will be extremely unlikely to "find" a schematic - the manufacturers will not wish casual repairs by users and the resulting litigation from cases of electrocution - they will probably restrict access to drawing sets to approved service agents.
You should not need a schematic to replace a triac - just the component and a soldering iron etc.
The sparking from underneath will probably not be caused by the triac which will either work or will not work and will hardly ever (never) give off sparks - I would suspect brush failure in the motor (£) or the motor itself (££££).
Good luck...
Peccavi
December 2009