G'day.
Yes, tips are a great resource of old lawnmowers that normal people believe are no longer worth anything. They are normally not worth anything to your average person as it would cost so much dough to fix if you got a shop to do it. If a shop had to replace the earth lead, most would replace the complete magneto set-up which is $100+. i have picked up many from hard-rubbish which have also been great.
But getting to my point, if you get a little practice, there is absolutely no need to take it to a mower shop for a tune-up. Replace the spark-plug if it looks **** , change the oil and clean the filter if it's one of the sponge-type ones. and tune the carby, then you'll be doing exactly what your average shop would do.
Sam.