I just got in the new Burris Gen III valve kit for the Yamaha and it works wonderfully in the GX200. We need to substitute a few of the parts with some others.
Here is what you need for this setup:
2 F21-408-00 rocker arms $9 ea
2 F21-405-11 intake springs $7.50 ea - yields approx 19lbs at the seat with plenty of room for shims
2 F21-417-00 lash caps $4 ea
2 F21-406-20 spring retainers $7 ea
2 F21-407-20 retainer lock set $7 ea
1 F21-402-25 stainless intake valve $31
1 F21-403-20 stainless exhaust valve $31
We carry .015" .030" and .060" shims @ $1.50 ea
I recommend the push rods from the clone engine, however, I do not have them yet.
The intake valve is 1.070" (.072" larger than stock) and you will want to either install or have installed a new large inside diameter intake seat, (p/n 6148Y $8) or have a 60° cut on the existing seat to open it up to take advantage of the larger valve.
You will have to open up the slots in the push rod guide plate on the GX200 if you go with the clone pushrods. If you are hopping up a clone, you may need to fabricate a new push rod guide plate.
This setup gives you a large intake valve, approx 30% more valve lift, and as much spring pressure potential as you could possibly need. You need to use a billet flywheel and a billet rod with this as well. I think the long rod and a JE EXF4500-120 piston would be the best setup there. I also think the PVL ignition, along with the matching ARC flywheel will be the hot ticket if you are going with methanol. I personally suggest calling EC Distributing for your carb and billet intake. I have a few intakes and some gas carbs. But when I need a rockin carb for methanol, I call Carroll Ford at EC.
With this setup, you will be taxing the limits of strength on both the crank and the block. Where the line will be will depend on a lot of things, but if you can't stomach blowing up a motor, you might want to let some of us who live for that sort of thing find the weaknesses and come up with solutions.