Some pictures would help, but usually rusty flywheel magnets are the culprit if it has set outside, or just set for a while. Remove the shroud covering the flywheel, and look for the magnets. Should be two, cast into the, I'm assuming aluminum flywheel, so they'll be easy to spot. Polish them with some fine, like 400 grit sandpaper. Or, I use my air angle die grinder with a Roloc pad for removing gaskets. Buffs them right up, with little work. I normally clean further to remove the dust with some brake cleaner, or a bit of WD-40 on a shop rag. I like to polish up under the coil pick-ups too with a single layer sheet of fine sandpaper, or crocus cloth.
If still no spark, could be the condenser, or points. Depending on the brand and model of the engine as to their location, you may need to remove the flywheel to get to them.
Again, a few pics from different angles would be a big help.