Well there are three areas I'd think of first... Fuel... Spark... Mechanical.
If you had a compression tester you could quickly eliminate any (very unlikely) issue there.
Very quick job to remove the spark plugs and see if they look OK... cheap to replace if there is any uncertainty over them.
But for me the most likely is a fuel issue of some kind so first I'd be checking the tank pickup, bulb and lines are fine then looking at the fuel filter and after that the carb/s for muck in the float bowl and/or jets.
After that probably consider a fuel pump issue.
And then if all the above have checked out I'd return to looking at the ign system considering the possibility of lead and/or coil pack/ign unit problems... rare in my experience though.