Finally the workshop got it's saying.
I would break it down like this.
1. Test tank run -change impeller and gear oil = 1+ Hour.
2. Off power head, off head, drill out bolts, clean, replace parts, re-test = 4 hours ++
Roughly 6-7 hrs work.
Then it all comes down to the workshop hour rate.
Some people have the misconception that small engines are cheaper to repair than bigger ones, not quite.
The mech (hopefully) has the same hourly pay whichever size engine he works on.
Given that the engine may not have been 'touched' since new, it gives an about yearly service cost of $50 (15 years old), which is not too bad at all!