Advice please 2005 80cc trials

[Kerabo]

Hello all
My first post.
I have been asked to have a look at a mates bike that suddenly stopped running. Although not a mechanic he has changed various electrical parts from one bike to another but it still won't start. He says a weak spark. I will look in a couple of days but just wondered if there are any weak parts that I should go to first?
Any advice appreciated.
Thanks Ken