Worth trying a new plug since they are cheap and easy. Some work out of the engine, but stop working when you install them. I've had that cause backfires with no start.
Try a timing light and see if you are getting spark at the right point.
I'm trying to figure out who even makes these forks so I can get a manual and parts list. They are 38mm. The bike is a 2008 txtpro, but who knows where the forks are from. Who's logo is that? I googled a few makers and can't find it.