From one X to another, welcome to the site.
Here's my Yerf thread with lots of info for you if you haven't seen it yet.
http://buggymasters.com/forum/showthread.php?t=2601
First things first. Reinforce the front end.
1. The Howhit engine that comes on it, is pretty good. The head in particular can be cleaned up and breathe much better. (FREE mod) Not a bad head IMO. You can have it ported and polished if you want.
2. A12 is good. Try to find a quality built one, not just cheapest you can find.
3. Not knocking either one, but a cleaned up stocker and opened up muffler will make decent improvements. (FREE mod)
4. Don't waste your $$. No major gains from either of them. (maybe the unit that has adjustable timing, but don't think you're going to see much improvement)
5. UNI good. Will need to rejet carb.
6. Stock carb not too bad. I did notice a gain from my flat slide Mikuni though
7. Trial and error to find what works best for you/son. This is where you can notice a big improvement in performance by getting this dialed in right.
8. Don't waste your $$$. Stock is just fine. That's what I run with no problem. Just deglaze the clutch bell with sand paper and you'll be fine. I also run stock variator after trying a $80 performance variator. (gained me nothing)
9. I modified mine. They came out good. I do however think Jersey Devil's car/trailer bearing spindle mod is the best set up.
NOTE: when I say cleaned up, I mean grinding and polishing the casting flaws out of parts.
Port matching the intake manifold to cylinder head, and head to exhaust manifold frees up a lot of usable power. It's awful how far off these can be and the engine still runs.