The 5/8's is very tame (like a mellow 4 stroke 90) and perfect to demonstrate when and why to use the clutch. It's on the slow side (with me on it) unless you shift a lot and find the right gear to get the rev's up. He even said it was a little slow, and "you'll put the bigger one in for the race, right?". So he is no longer afraid of the bike at all and is working on his slow speed turns while using the clutch. I couldn't be happier with this mod. I would do the flywheel weight to help with the woods riding, but only found them for $129 and will stick with the free mods for now.
I also made some new dog bones for the rear suspension and made them about 89mm (longest of 4 stock sizes was 88.3mm). It was pretty easy to do with some scrap steel, cut to size with a 4.5" angle grinder or portable bandsaw, and make holes with a drill press (a Harbor freight 12 speed bench top drill press on slowest speed). I raised the forks up in the triple-clamp until they almost touched the handlebars. He was able to start the bike yesterday (once) while standing on the ground. He's not quite able to touch both riding boots down at the same time, can touch one flat footed by sliding off the seat a little. Found someone who made them 95mm long (they said +1mm= -1/4" rear fender height), and might whip up an extra set at 93mm of these to try. During his hair scrambles race, they don't do any jumping, so not too worried about tire/fender clearance. But his ability to start the bike in the woods is a concern for me, so might lower it a little more.