I have a generator with a 10 HP Briggs & Stratton OHV engine. Turns over when pulled but won't fire. Replaced all fuel, cleaned carb bowl (no water in fuel). Has weak but intermittant spark.

Flywheel has very slight coat of rust, plug is clean and fuel getting to cylinder. The whole machine has barely had 20 hours of run time and has been stored ion a clean dry location.

Any ideas what it could be and how to correct?

Take some 400 grit sandpaper and get rid of the rust. If it interferes with the gap it will interfere with firing.

Yes, clean the flywheel and use a business card to set the gap between the coil and the flywheel.