Checking the piston to see if it is siezed is easy..
If we are talking about a Tomos, try to kick start it.. If the pedals are able to go backwards it is not seized.
Any other ped with a decomp. lever Try to start it with lever pulled in... if pedals won't move, it has seized.
Now if it still kicks over but wont start, it must be something else.
But if it has seized.. It is very tricky to fix. If you don't know how, I would sudjest finding profesional help. You have to rehone the cylinder, Install a new piston. Ect.. It is difficult using a hone, so I myself would not try it. See a profesional If it seems to be siezed.