I'm assuming you checked the spark with a fresh plug? A fouled plug won't spark strongly, if at all. After that, check and clean all the connections, and make sure they're clean and making solid contact. Check your grounds, including the one at the coil. If that doesn't do it, you're going to need to get the specs for the stator, coil and CDI from a service manual, and check those. It could be a stator, CDI or external coil going bad, but until you check it, you can't say for sure. Electrical parts are expensive, and can't be returned, so check the free things first before buying parts you may not need.