if the oil has been changed regular and always on the level pretty much rule out scored bore...say that though it wants to start when it cooled down could mean its a problem there :S

if you got spark its not the electronic unit.

tbh i doubt the headgasket has gone on it.

the crank (shaft from engine out) isnt bent is it?

have you tried starting it with out the filter on? try that.

if not one of the first things i used to do is clean the carb out with an airline or a hard blow just make sure you keep your finer over the middle hole as some carbs lose the little gasket with a hard blow....and you wont find it...trust me them carbs are so small a little bit of shit in it and they piss you around