the quick fix would probably be to do some experimentation with sprockets. First gear might become totally useless, but it might tighten up the rest to make it useable.
the other option might be to see if the local bike shops have any grenaded 250R engines laying around that you can cannibalize, then just lay everything out and see if things might work. I'd say look at the pictures on the fiche, but those aren't always dead on.
You might check with Service Honda, because they more than likely have plenty of 250R engines they remove to build their 500AF.